CI: use ubuntu-18.04 instead of 20.04.

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ on:
 jobs:
   linux:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: true
       matrix:

.github/workflows/nightly.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   linux:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v1
     - name: Install dependencies

.github/workflows/release-candidate.yml

@@ -8,7 +8,7 @@ on:
 jobs:
   linux:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@v1
     - name: Create linux package