Pinned CI runner versions

Author: s1hofmann

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ jobs:
   test:
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-20.04, macos-11, windows-2019 ]
         node: [ 16 ]
     runs-on: ${{matrix.os}}
     steps:

.github/workflows/snapshot_release.yaml

@@ -8,7 +8,7 @@ jobs:
   test:
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-20.04, macos-11, windows-2019 ]
     runs-on: ${{matrix.os}}
     steps:
       - name: Set up Git repository
@@ -40,7 +40,7 @@ jobs:
       - test
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-20.04, macos-11, windows-2019 ]
     runs-on: ${{matrix.os}}
     steps:
       - name: Set up Git repository

.github/workflows/tagged_release.yaml

@@ -8,7 +8,7 @@ jobs:
   test:
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-20.04, macos-11, windows-2019 ]
     runs-on: ${{matrix.os}}
     steps:
       - name: Set up Git repository
@@ -40,7 +40,7 @@ jobs:
       - test
     strategy:
       matrix:
-        os: [ ubuntu-latest, windows-latest, macos-latest ]
+        os: [ ubuntu-20.04, macos-11, windows-2019 ]
     runs-on: ${{matrix.os}}
     steps:
       - name: Set up Git repository