chore(shim): make dynamic downloads opt in (#8458)
### Description
Until we properly document and advertise the feature we'll have this
opt-in. Adds `TURBO_DOWNLOAD_LOCAL_ENABLED` in favor of
`TURBO_DOWNLOAD_LOCAL_DISABLED`
### Testing Instructions
Manual testing that env vars are respected
```
[0 olszewski@chriss-mbp] /tmp/more-lefthook $ turbo_dev --version -vv
2024-06-12T11:41:28.918-0700 [DEBUG] turborepo_lib::shim: Global turbo version: 2.0.4-canary.2
2024-06-12T11:41:28.919-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/tmp/more-lefthook/package.json
2024-06-12T11:41:28.925-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/tmp/package.json
2024-06-12T11:41:28.925-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/package.json
2024-06-12T11:41:28.925-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /package.json
2024-06-12T11:41:28.925-0700 [DEBUG] turborepo_lib::shim: Repository Root: /private/tmp/more-lefthook
2024-06-12T11:41:28.926-0700 [DEBUG] turborepo_lib::shim::local_turbo_state: No local turbo binary found at: /private/tmp/more-lefthook/node_modules/turbo-darwin-arm64/bin/turbo
2024-06-12T11:41:28.926-0700 [DEBUG] turborepo_lib::shim::local_turbo_state: No local turbo binary found at: /private/tmp/more-lefthook/node_modules/turbo/node_modules/turbo-darwin-arm64/bin/turbo
2024-06-12T11:41:28.926-0700 [DEBUG] turborepo_lib::shim::local_turbo_config: downloading correct local version not enabled
2024-06-12T11:41:28.926-0700 [DEBUG] turborepo_lib::shim: Running command as global turbo
WARNING No locally installed `turbo` found. Using version: 2.0.4-canary.2.
2.0.4-canary.2
[0 olszewski@chriss-mbp] /tmp/more-lefthook $ TURBO_DOWNLOAD_LOCAL_ENABLED=1 turbo_dev --version -vv
2024-06-12T11:41:37.467-0700 [DEBUG] turborepo_lib::shim: Global turbo version: 2.0.4-canary.2
2024-06-12T11:41:37.467-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/tmp/more-lefthook/package.json
2024-06-12T11:41:37.470-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/tmp/package.json
2024-06-12T11:41:37.470-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/package.json
2024-06-12T11:41:37.470-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /package.json
2024-06-12T11:41:37.470-0700 [DEBUG] turborepo_lib::shim: Repository Root: /private/tmp/more-lefthook
2024-06-12T11:41:37.470-0700 [DEBUG] turborepo_lib::shim::local_turbo_state: No local turbo binary found at: /private/tmp/more-lefthook/node_modules/turbo-darwin-arm64/bin/turbo
2024-06-12T11:41:37.470-0700 [DEBUG] turborepo_lib::shim::local_turbo_state: No local turbo binary found at: /private/tmp/more-lefthook/node_modules/turbo/node_modules/turbo-darwin-arm64/bin/turbo
2024-06-12T11:41:37.492-0700 [DEBUG] turborepo_lib::shim: Found configuration for turbo version 2.0.3
2024-06-12T11:41:37.492-0700 [DEBUG] turborepo_lib::shim: Running turbo@2.0.3 via npx
2024-06-12T11:41:37.492-0700 [DEBUG] turborepo_lib::shim: supports_skip_infer_and_single_package true
2024-06-12T11:41:38.068-0700 [DEBUG] turborepo_lib::shim: Global turbo version: 2.0.3
2.0.3
[0 olszewski@chriss-mbp] /tmp/more-lefthook $ TURBO_DOWNLOAD_LOCAL_ENABLED=0 turbo_dev --version -vv
2024-06-12T11:41:41.123-0700 [DEBUG] turborepo_lib::shim: Global turbo version: 2.0.4-canary.2
2024-06-12T11:41:41.123-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/tmp/more-lefthook/package.json
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/tmp/package.json
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /private/package.json
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_repository::package_json: loading package.json from /package.json
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_lib::shim: Repository Root: /private/tmp/more-lefthook
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_lib::shim::local_turbo_state: No local turbo binary found at: /private/tmp/more-lefthook/node_modules/turbo-darwin-arm64/bin/turbo
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_lib::shim::local_turbo_state: No local turbo binary found at: /private/tmp/more-lefthook/node_modules/turbo/node_modules/turbo-darwin-arm64/bin/turbo
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_lib::shim::local_turbo_config: downloading correct local version not enabled
2024-06-12T11:41:41.125-0700 [DEBUG] turborepo_lib::shim: Running command as global turbo
WARNING No locally installed `turbo` found. Using version: 2.0.4-canary.2.
2.0.4-canary.2
```
