CI: Use ubuntu-20.04 instead of ubuntu-latest (#3668)

Author: rbuch

.github/workflows/buildold.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     timeout-minutes: 90
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/changa.yml

@@ -4,7 +4,7 @@ on: pull_request
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/charm4py.yml

@@ -4,7 +4,7 @@ on: pull_request
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/cuda.yml

@@ -8,7 +8,7 @@ jobs:
   build:
     timeout-minutes: 45
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v1

.github/workflows/doc.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   sphinx-build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/mpi_linux_smp.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     timeout-minutes: 60
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/namd.yml

@@ -7,7 +7,7 @@ jobs:
   # Since NAMD requires a secret to download its source from Gitlab, CI builds from outside PPl
   # always fail in this test since the secret is not available.
   build-and-test-namd:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/netlrts_linux_i386.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     timeout-minutes: 60
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/shellcheck.yml

@@ -9,7 +9,7 @@ jobs:
   build:
     timeout-minutes: 45
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/spack.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-20.04, macos-latest]
 
 
     steps: