Do not let cc-rs trigger a rebuild any time the env changes

[hintron]

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[hintron]

My main goal with this PR is to fix the annoying re-compile/re-check times for Bevy 0.14 on Windows. Bevy 0.14 uses BLAKE3 under the hood (since 0.12, I believe).

cc-rs (which BLAKE3 uses) I think started getting more aggressive at detecting env changes in v1.1.0 to the point where even PATH changes caused cc-rs to want to rebuild, and so that's probably why I started noticing the issue after upgrading to Bevy 0.14.

Luckily, there is a setting to turn off env change rebuilds in cc-rs (emit_rerun_if_env_changed). This should leave it up to BLAKE3 to determine in build.rs what envs are important enough to trigger a rebuild for.

For more analysis of what is going on and what I found when digging through the code, see #324 (comment).

[hintron]

Testing this PR results in just the following rerun-if-env-changed commands being emitted for me on x64 WIndows 11:

cargo:rerun-if-env-changed=CARGO_FEATURE_PURE
cargo:rerun-if-env-changed=CARGO_FEATURE_NO_NEON
cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
cargo:rerun-if-env-changed=CRATE_CC_NO_DEFAULTS
cargo:rerun-if-env-changed=CFLAGS_x86_64-pc-windows-msvc
cargo:rerun-if-env-changed=CFLAGS_x86_64_pc_windows_msvc
cargo:rerun-if-env-changed=HOST_CFLAGS
cargo:rerun-if-env-changed=CFLAGS
cargo:rerun-if-env-changed=CARGO_FEATURE_PREFER_INTRINSICS
cargo:rerun-if-env-changed=CARGO_FEATURE_PURE
cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
cargo:rerun-if-env-changed=CARGO_FEATURE_PURE
cargo:rerun-if-env-changed=CARGO_FEATURE_PREFER_INTRINSICS
cargo:rerun-if-env-changed=CC_ENABLE_DEBUG_OUTPUT
cargo:rerun-if-env-changed=CARGO_FEATURE_NEON
cargo:rerun-if-env-changed=CARGO_FEATURE_NO_NEON
cargo:rerun-if-env-changed=CARGO_FEATURE_PURE
cargo:rerun-if-env-changed=CC
cargo:rerun-if-env-changed=CFLAGS

(the duplicate emissions are a benign issue, but maybe we can clean that up later)

That contains 4 of the 5 envs explicitly checked for in BLAKE's build.rs:

• CARGO_FEATURE_PURE
• CARGO_FEATURE_PREFER_INTRINSICS
• CARGO_FEATURE_NEON
• CARGO_FEATURE_NO_NEON

All the other env check commands (CC, CFLAGS, etc.) are still coming from cc-rs, I think, but that's a much better situation than before, where PATH and LIB and all sorts of envs were emitted.

Now, when I make a change to my Bevy game in VSCode with Rust Analyzer (causing cargo check to run in the background), I can subsequently run cargo check in my terminal without triggering a recompile!