A way for tests to look up `file!` paths at runtime

[epage]

Problem

In tests, I want to be able to

• Update snapshots in the test's source
• Read or write to files from a path relative to the test

Proposed Solution

Provide the base path for file! as an environment variable

Note: Rust 1.89 specified the base path for file!

Notes

Split out of #3946

[epage]

We previously had CARGO_RUSTC_CURRENT_DIR (#12996) but that was before file! was specified. This was set to an absolute path on every crate at compile time.

We had considered limiting it to test and bench when it was a different mechanism we were exposing.

[epage]

We talked about this at today's cargo team meeting

There was confusion on whether this was set at compile time or runtime. It is compile time which has the downside of requiring a rebuild when it changes (likely in CI) but other variables have the same problem like CARGO_TARGET_TMPDIR and CARGO_BIN_<name>, so nothing new there. If we made this available at runtime, it might not work out the same since dependencies can read your value. This also puts more of a burden on people running tests outside of cargo test for which we would need to document it (same if we decide to make CARGO_BIN_<name> available at runtime).