[build-script] Prioritize SDK paths in CMake find_ functions

[kastiglione]

Fixes a build failure that happens on macOS when both the CommandLineTools and homebrew pkg-config are installed.

Homebrew's pkg-config is setup to hardcode CommandLineTools paths, which causes many issues. As a result, when find_package() consults pkg-config, targets can have paths containing CommandLineTools, which can result in non-obvious build failures. Two example packages are libedit and libxml2.

This fix assumes that, as a default, xcrun --sdk macosx --show-sdk-path should be preferred to pkg-config.

The CMake documentation shows the search order that the find_ functions use (see find_path). This change makes use of CMAKE_PREFIX_PATH as an environment variable to prioritize the SDK sysroot over pkg-config. This is the 3rd search pass, just above the HINTS search pass (which is what's used by packages such as FindLibEdit.cmake).

If needed, to explicitly build with a custom package, the documentation shows that higher priority search passes are available, such as setting -DCMAKE_PREFIX_PATH=....

Setting the environment variable is also composable with a user supplied CMAKE_PREFIX_PATH env variable.

Resolves SR-12726.

[JDevlieghere]

@swift-ci please test

[AndrewSB]

could we merge this please? just spent half an hour debugging the problem that this fixes

[JDevlieghere]

@kastiglione any objections to merging this?

[kastiglione]

No objections, thanks for reviewing and merging.