Some packages (e.g. thiserror) force a recompile if the value of the `RUSTC_BOOTSTRAP` env var changes. RA sets the variable to 1 in order to enable rustc / cargo unstable options it uses. This causes flapping recompiles when building outside of RA.
As of Cargo 1.75 the `--keep-going` flag is stable. This change uses the flag without `RUSTC_BOOTSTRAP` if the Cargo version is >= 1.75, and drops `--keep-going` otherwise. This fixes build script recompilation.
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