Allow bootstrap cache path to be set by environment variable

[jefferyto]

This allows the bootstrap cache path to be set by the RUSTC_BOOTSTRAP_CACHE environment variable.

Setting the bootstrap cache path to an external location can help to speed up builds in cases where the build directory is not kept between builds, e.g. in CI or other automated build systems.

[rustbot]

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[jefferyto]

FYI we at the OpenWrt packages feed recently merged a patch essentially the same as this one (openwrt/packages#22339).

[onur-ozkan]

Instead of RUSTC_BOOTSTRAP_CACHE env, it would be more appropriate to add a new field(something like bootstrap-cache) to the build configuration under the [build] section. That way we can document it too in config.example.toml file.

[bors]

☔ The latest upstream changes (presumably #116196) made this pull request unmergeable. Please resolve the merge conflicts.

[jieyouxu]

cc @jefferyto from triage: marking this PR as waiting-for-author due to merge conflicts.
@rustbot author

[onur-ozkan]

This will be implemented with #121976