Downgrade to ubuntu 18.04 to fix buildah jobs

This fixes our builds.

Related-to: redhat-actions/buildah-build#45

Signed-off-by: Juan Antonio Osorio Robles <jaosorior@redhat.com>

.github/workflows/build_image.yaml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 jobs:
   build-fedora:
     name: Build Fedora image
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
     - uses: actions/checkout@v2
@@ -19,7 +19,7 @@ jobs:
 
   build-centos:
     name: Build CentOS image
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
     - uses: actions/checkout@v2
@@ -30,4 +30,4 @@ jobs:
         image: selinuxd-centos
         tags: latest ${{ github.sha }}
         dockerfiles: |
-          ./images/Dockerfile.centos
+          ./images/Dockerfile.centos

.github/workflows/e2e.yml

@@ -6,7 +6,7 @@ on:
   pull_request:
 jobs:
   e2e-image:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - run: make fedora-image