CI: Add a temporary workaround for broken MSYS2 Python path separator behaviour

"MSYSTEM= python ..." no longer works because of some changes in the MSYS2 runtime

Until this is fixed in either MinGW Python or the MSYS2 runtime this should revert
things to the previous behaviour to get the CI green again.

.github/workflows/msys2.yml

@@ -51,6 +51,9 @@ jobs:
             TOOLCHAIN: clang
     env:
       MESON_CI_JOBNAME: msys2-${{ matrix.NAME }}
+      # XXX: Otherwise unsetting MSYSTEM doesn't work
+      # https://github.com/msys2/msys2-runtime/pull/101#issuecomment-1255195675
+      MSYS: noemptyenvvalues
 
     defaults:
       run: