ci: Favor SCRIPT instead of RUST_CHECK_TARGET

ci: Favor SCRIPT instead of RUST_CHECK_TARGET

Since #61212 we've been timing out on OSX, and this looks to be because
we're building tools like Cargo and the RLS twice instead of once. This
turns out to be a slight bug in our configuration. CI builders using the
`RUST_CHECK_TARGET` directive actually execute `make all` just before
their acual target. In `make all` we're building a stage2 cargo, and
then in `make dist` we're building a stage1 cargo.

Other builders use `SCRIPT` which provides explicit control over what
`x.py` script, for example, is used to execute the build. This moves
almost all targets to using `SCRIPT` to ensure that we're explicitly
specifying what's being built where. Additionally this updates the logic
of `RUST_CHECK_TARGET` to remove the pre-flight tidy as well as the
pre-flight `make all`. The system LLVM builder (run on PRs) now
explicitly runs tidy first and then runs the rest of the test suite.