ci: only use apt during testing

Real container pushing happens on a device where QEMU is already
installed.

Signed-off-by: Paul Spooren <mail@aparcar.org>

.github/workflows/containers.yml

@@ -358,6 +358,7 @@ jobs:
 
     steps:
       - name: Set up QEMU
+        if: github.event_name == 'pull_request'
         run: |
           sudo apt-get update
           sudo apt-get install -y qemu-user-static binfmt-support