CI: Set up msys2 build again
Hack installer_msys2/Makefile so that CI build will pass even with environment (from setup-msys2, latest version, #42892) partly incompatible with our official msys2 snapshot.
Hack installer_msys2/Makefile so that CI build will pass even with environment (from setup-msys2, latest version, #42892) partly incompatible with our official msys2 snapshot.