tools.PkgConfig: honor PKG_CONFIG env var - #9627

This environment variable is used at least by:

• cmake https://github.com/Kitware/CMake/blob/2a4a630f3acc83cd8eed95f940b54a851f9722d6/Modules/FindPkgConfig.cmake#L33
• meson https://github.com/mesonbuild/meson/blob/a9e9b7c7501a3c8a5984a93879d1f309bf8c72aa/docs/markdown/Release-notes-for-0.37.0.md#overriding-more-binaries-with-environment-variables
• autotools https://autotools.io/pkgconfig/pkg_check_modules.html#pkgconfig.pkg_check_modules.optional

Changelog: Feature: PkgConfig helper now honors PKG_CONFIG environment variable.
Docs: Omit

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