CI: update Ubuntu image to 20.04

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
         path: libarchive.tar.xz
 
   Ubuntu:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         bs: [autotools, cmake]
@@ -83,7 +83,7 @@ jobs:
         name: libarchive-ubuntu-${{ matrix.bs }}-${{ matrix.crypto }}-${{ github.sha }}
         path: libarchive.tar.xz
   Ubuntu-distcheck:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@master
     - name: Install dependencies