zfs-2.1.7: Use ubuntu-20.04 for zloop and sanity builders

The zfs-2.1.7 branch is still using the older 'python-dev'
package names rather than the newer 'python3-dev' packages that
are required for 'ubuntu-latest'.  Use 'ubuntu-20.04' instead of
'ubuntu-latest' to get around this.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>

.github/workflows/zfs-tests-sanity.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
       with:

.github/workflows/zloop.yml

@@ -6,7 +6,7 @@ on:
 
 jobs:
   tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       TEST_DIR: /var/tmp/zloop
     steps: