Rollup merge of #138395 - Kobzol:ci-download-gcc, r=Mark-Simulacrum

Download GCC from CI on test builders

This should reduce the duration of the `x86_64-gnu-llvm-18` job, which runs on PR CI, which is currently the only one that builds GCC (outside of the x64 dist builder).

Since we handle the GCC download in the GCC step, and not eagerly in config, we can set this flag globally across all test builders, as it won't do anything unless they actually try to build GCC.

Opening as a draft to test if it works on CI, because I still need to implement logic to avoid the download if there are any local modifications to GCC (essentially the "if-unchanged" mode, although I want to try something a bit different).

r? ```@ghost```

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