github actions: Upgrade test OS to 24.04

20.04 has a problem where podman is not compatible with docker, causing
tests to fail

.github/workflows/main.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   shellcheck:
     name: Shellcheck
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v2
 
@@ -33,7 +33,7 @@ jobs:
           SHELLCHECK_OPTS: -s bash -e SC1090 -e SC2181
 
   check:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v2
 
@@ -81,7 +81,7 @@ jobs:
           - {image: ubuntu-22.04-oe,        provider: podman, sh: bash}
           - {image: ubuntu-22.04-oegarmin,  provider: docker, sh: bash}
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     env:
       USE_HELPER: "1"
@@ -92,7 +92,7 @@ jobs:
 
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.6
+          python-version: 3.6.15
 
       - name: Prepare build
         run: ./ci/prepare.sh