GHA: Raspbian: install more recent QEMU

to workaround "semop(1): encountered an error: Function not implemented".

.github/workflows/raspbian.yml

@@ -33,6 +33,10 @@ jobs:
       - name: qemu-user-static
         run: |
           set -exo pipefail
+          . /etc/os-release
+          if [ "$VERSION_ID" = 20.04 ]; then
+            sudo perl -pi -e s/focal/impish/g /etc/apt/sources.list
+          fi
           sudo apt-get update
           DEBIAN_FRONTEND=noninteractive sudo apt-get install -y qemu-user-static