Related to conan-io/conan-center-index#2743, conan-io/setup-conan#4 and #16963 (comment)

When consuming the Conan GH action setup-conan, and trying to build ffmpeg from source, it will require the tool package msys2 as well, and a separated package for pkg-config tool.

The setup-conan uses setup-python to install Conan client, which injects PKG_CONFIG_PATH using Windows path style. At same time, Conan client, when running pkg-config, prepends the generator folder path using unix path, the supported one. The result is a mixed path style in the PKG_CONFIG_PATH which is not supported by pkg-config: https://www.msys2.org/docs/pkgconfig/#syntax-paths-escaping

The environment variable looks like:

PKG_CONFIG_PATH='/c/users/runneradmin/.conan2/p/b/ffmpef3af080da2c28/b/build-release/conan:C:\hostedtoolcache\windows\Python\3.10.11\x64/lib/pkgconfig:/mingw64/lib/pkgconfig:/mingw64/share/pkgconfig'

This PR tries to reproduce the known case in Conan client tests.

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