[build-script] Disable pkg-config for Darwin hosts (NFC)

[kastiglione]

Disable pkg-config on Darwin to prevent compilation issues.

This is an NFC follow up to #32436 and #32437.

The benefits are:

1. Prevents the pkg-config issues directly, instead of a roundabout way
2. Centralize two related pieces of build isolation config to a single place

For 1, the original issue was that CMake find_ functions would search pkg-config before the SDK. Homebrew pkg-config hardcodes CommandLineTools, even when Xcode is installed, and this caused problems. See SR-12726.

Disabling pkg-config gives better isolation, and it removes the use of CMAKE_PREFIX_PATH which I see as a bigger hammer than disabling pkg-config.

Before any of these changes, if pkg-config is installed, the effective search order was:

1. Try CMAKE_PREFIX_PATH (empty)
2. Try HINTS paths (CommandLineTools via pkg-config)
3. Try SDK paths

After #32436, the relevant search order became:

1. Try CMAKE_PREFIX_PATH (SDK)
2. Try HINTS paths (CommandLineTools via pkg-config)
3. Try SDK paths

Now with this change, the relevant search order is:

1. Try CMAKE_PREFIX_PATH (empty)
2. Try HINTS paths (empty)
3. Try SDK paths

[gottesmm]

Is there a cmake option we can use that just does this without us needing to hack around it?

@compnerd

[kastiglione]

I spoke with @gottesmm, he's referring to the line below:

-DPKG_CONFIG_EXECUTABLE=/usr/bin/true

[kastiglione]

This was discussed out of band, and the conclusion was that disabling pkg-config via PKG_CONFIG_EXECUTABLE=/usr/bin/false is the most direct & concise way to do this. A larger scoped solution is to use checked-in CMake cache files to explicitly declare SDK dependencies. This could approach could happen in the future.

[gottesmm]

LGTM

[kastiglione]

@swift-ci please smoke test and merge