GHA: unset SUDO env vars when calling mkosi build

Otherwise mkosi tries to change user and fails

(cherry picked from commit 067dac4)

Author: bluca

.github/workflows/mkosi.yml

@@ -131,7 +131,7 @@ jobs:
       run: sudo mkosi genkey
 
     - name: Build ${{ matrix.distro }}
-      run: sudo mkosi --repository-key-check=false
+      run: sudo env -u SUDO_USER -u SUDO_UID -u SUDO_GID mkosi --repository-key-check=false
 
     - name: Show ${{ matrix.distro }} image summary
       run: sudo mkosi summary