Run GitHub Actions under Ubuntu 20.04

.github/workflows/enclave.yml

@@ -9,7 +9,7 @@ on:
 
 jobs:
   make_test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
       - name: Docker cache
@@ -20,7 +20,7 @@ jobs:
       - run: make docker_"check clippy test"
         working-directory: enclave
   make_docker:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
       - name: Docker cache

.github/workflows/service.yml

@@ -7,14 +7,14 @@ on:
 
 jobs:
   make_all:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
       - uses: ./.github/actions/service
         with:
           target: all
   make_test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
       - uses: ./.github/actions/service