[test] Isolate build-script calls in skip-local-build.test-sh

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

Author: edymtt

validation-test/BuildSystem/skip-local-build.test-sh

@@ -1,8 +1,10 @@
 # REQUIRES: standalone_build
-#
-# RUN: %swift_src_root/utils/build-script --dump-config --skip-local-build 2>&1 | %FileCheck %s -check-prefix=CONFIG
+
+# RUN: %empty-directory(%t)
+# RUN: SWIFT_BUILD_ROOT=%t %swift_src_root/utils/build-script --cmake %cmake --dump-config --skip-local-build 2>&1 | %FileCheck %s -check-prefix=CONFIG
 # CONFIG: "skip_local_build": true
 
-# RUN: %swift_src_root/utils/build-script --dry-run --verbose-build --skip-local-build 2>&1 | %FileCheck %s -check-prefix=DRY
+# RUN: %empty-directory(%t)
+# RUN: SWIFT_BUILD_ROOT=%t %swift_src_root/utils/build-script --cmake %cmake --dry-run --verbose-build --skip-local-build 2>&1 | %FileCheck %s -check-prefix=DRY
 # DRY: build-script-impl
 # DRY-SAME: --skip-local-build