Remove allow_failure from the msys2-mingw64 job when pkg-config-rs gets fixed
https://github.com/rust-lang/pkg-config-rs/issues/150 is causing this in the msys2-mingw64 job:
cargo:warning=`"C:/msys64/mingw64/bin/pkg-config" "--modversion" "glib-2.0" "glib-2.0 >= 2.56"` did not exit successfully: exit code: 1
--- stderr
pkgconf: --modversion requested with multiple packages, output would be ambiguous
I've turned on allow_failure
in the job for now; let's remove it when the pkg-config-rs bug is fixed.