pallet-revive: Use `RUSTUP_TOOLCHAIN` if set (#6365)
We were not passing through the `RUSTUP_TOOLCHAIN` variable to the
`build.rs` script of our fixtures. This means that setting the toolchain
like `cargo +1.81 build` had no effect on the fixture build. It would
always fall back to the default toolchain.
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Co-authored-by: GitHub Action <[email protected]>