ci/mingw: remove cache

To avoid building against stale version of dependencies. In particular
libplacebo is moving target and as we can see the build has been broken
few times recently, so let the CI validate it for us.

The time to build everything is under 30 minutes, which is acceptable in
my opinion, not much longer than macos build.

.github/workflows/build.yml

@@ -18,12 +18,6 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      # Increase -N suffix here to force full rebuild after changes
-      - uses: actions/cache@v3
-        with:
-          path: mingw_prefix/
-          key: "${{ matrix.target }}-7"
-
       - name: Install dependencies
         run: |
           sudo apt-get update