Rebuild if deployment target changes

rustc reads the MACOSX_DEPLOYMENT_TARGET environment variable (and similar other *_DEPLOYMENT_TARGET variables), and uses that value to determine (probably amongst other things) certain options for LLVM and the linker.

However, it seems like this environment variable is not tracked as an external dependency, meaning that changing it does not cause rustc/cargo to rebuild the target, as is otherwise expected when changing environment variables that modify compilation.

E.g. running the following code:

cargo clean
MACOSX_DEPLOYMENT_TARGET=12.0 cargo build
MACOSX_DEPLOYMENT_TARGET=13.0 cargo build

I expected to see this happen: The project was built twice.

Instead, this happened: The project was built once, and then the cache was used the second time.

Meta

rustc --version --verbose:

rustc 1.76.0-nightly (2f8d81f9d 2023-11-21)
binary: rustc
commit-hash: 2f8d81f9dbac6b8df982199f69da04a4c8357227
commit-date: 2023-11-21
host: aarch64-apple-darwin
release: 1.76.0-nightly
LLVM version: 17.0.5

Start with compiler/rustc_target/src/spec/base/apple/mod.rs at the environment-variable handling referenced in the issue, then trace how deployment-target variables become external build dependencies. Reproduce the behavior with the two MACOSX_DEPLOYMENT_TARGET cargo builds. Done means changing the deployment target causes the target to rebuild, with regression coverage for the demonstrated sequence.