workflows: use Ubuntu 20.04 instead of 18.04

18.04 is a bit old.

.github/workflows/builds.yml

@@ -12,7 +12,7 @@ on:
 jobs:
   build_cli:
     name: Build CLI
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v2
@@ -45,7 +45,7 @@ jobs:
   build_image:
     needs: build_cli
     name: Build Docker image
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v2

.github/workflows/publish_to_dockerhub.yml

@@ -27,7 +27,7 @@ on:
 jobs:
   publish:
     name: Publish image to DockerHub
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout (manual run)
         if: ${{ github.event_name == 'workflow_dispatch' }}

.github/workflows/tests.yml

@@ -23,7 +23,7 @@ jobs:
 
   cover:
     name: Coverage
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     env:
       CGO_ENABLED: 0
@@ -58,7 +58,7 @@ jobs:
 
   tests:
     name: Tests
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         go_versions: [ '1.16' ]