Switch Github runner from ubuntu-latest to ubuntu-20.04

We use the bundled qemu binaries at least for the E2E tests
which are running on Cirrus under Ubuntu-20.04, and the latest
binaries don't seem to work.

Signed-off-by: Jan Dubois <jan.dubois@suse.com>

Author: jandubois

.github/workflows/release.yml

@@ -133,7 +133,7 @@ jobs:
         if-no-files-found: error
 
   linux-build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 60
     steps:
     - uses: actions/setup-go@v2