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.

crates/uv-python/src/lib.rs

@@ -224,7 +224,7 @@ mod tests {
                     Some(self.installations.root().as_os_str()),
                 ),
                 // Set a working directory
-                ("PWD", Some(self.workdir.path().as_os_str())),
+                (EnvVars::PWD, Some(self.workdir.path().as_os_str())),
             ];
             for (key, value) in vars {
                 run_vars.push((key, *value));