test: Add end-to-end Go prune and cache coverage (#13962)
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### Description
Adds realistic Go end-to-end coverage for mixed repositories, dependency
isolation, local replacements, cache invalidation, checkout-stable
hashes, and executable pruned workspaces.
Closes TURBO-5965
### Testing Instructions
```bash
cargo test -p turbo --test go_workspace_test test_mixed_workspace_executes_and_caches_javascript_and_go_builds -- --exact --nocapture
cargo test -p turbo --test go_workspace_test test_go_hash_is_stable_across_equivalent_checkout_roots -- --exact --nocapture
cargo test -p turbo --test go_workspace_test test_go_cache_invalidates_every_north_star_input -- --exact --nocapture
cargo test -p turbo --test go_workspace_test test_go_prune_produces_minimal_valid_workspace -- --exact --nocapture
```
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
