It makes sense to just set this here since we aren't testing that functionality
and enables us to test this if we aren't using one of the specified Xcodes
(which can be useful).
This will align skip-local-build.test-sh with the behaviour of the
other BuildSystem tests, by
* ensuring we use the cmake exposed in lit.cfg, so that under Linux we
don't attempt to rebuild it
* using a separate build folder for build-script invocations, so that
side effects will not affect the main invocation and other lit tests.
I expect these changes to prevent llvm-targets-options.test to fail in
Linux presets with an error related to cmake, e.g.
```
build-script: error: argument --cmake:
/home/buildnode/jenkins/workspace/
oss-swift-package-linux-ubuntu-18_04/build/cmake-linux-x86_64/bin/cmake
is not an executable
```
Addresses rdar://78320684
