Do not unconditionally succeed RUSTC_WRAPPER checks when run by build scripts

rust-analyzer's RUSTC_WRAPPER unconditionally succeeds `cargo check`
invocations tripping up build scripts using `cargo check` to probe for
successful compilations. To prevent this from happening the RUSTC_WRAPPER
now checks if it's run from a build script by looking for the
`CARGO_CFG_TARGET_ARCH` env var that cargo sets only when running build
scripts.

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