default-crate-overrides: proc-macro-crate assumes env::var("CARGO")

Since version 2.0.0, proc-macro-crate has assumed it can exec()
`env::var("CARGO")` in order to run `cargo locate-project`.

This commit adds a crate override to proc-macro-crate which simply writes the
path to buildPlatform.cargo into the proc-macro-crate sources.

This way we don't need to set `env.CARGO` for every build that depends on
proc-macro-crate -- if we do that, the $CARGO environment variable would be
visible to the entire build.  This could potentially lead to incredibly
hard-to-troubleshoot heisenbugs if there is some other crate that expects
`env::var("CARGO")` to exist -- that other crate would mysteriously work only
in projects that use proc-macro-crate but not anywhere else!

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