chore(🧹): const env var usage cleanup (#16950)
## Summary
* Updates existing references to use EnvVars where usage was missing.
* Adds missing entries to env var usages, e.g. new env var declarations
in uv-trampoline, tests, etc.
* Note: this doesn't affect trampoline sizes as the end result is the
same
* Fixes versioning of `UV_HIDE_BUILD_OUTPUT`.
## Test Plan
Existing Tests. Compiled the trampolines locally to verify zero changes
(size, binary).
## Question
Will this complicate the crates publishing release process? I'm not
certain yet if it will be an issue for uv-trampoline (non-workspace
member) to reference a uv workspace member from a bump & release
perspective wrt lock files. If so, I'll revert the uv-trampoline changes
but keep the others.
