Default to Ubuntu 20.04 if OS is not specified

This makes the matrix more compact, as we don't need to specify the OS
for the majority of the field (all of them now, but we'll add builds
using "macos-N" later, so it will still be necessary for some of them),
and the builds more stable than using which the current latest Ubuntu
version is.

.github/workflows/ci.yml

@@ -15,7 +15,8 @@ on:
 jobs:
   build:
 
-    runs-on: ${{ matrix.os }}
+    runs-on: ${{ matrix.os || 'ubuntu-20.04' }}
+
     strategy:
       matrix:
         #SWIGJOBS: ["-j2"]
@@ -27,18 +28,14 @@ jobs:
         # PY3,VER
         # SWIG_FEATURES
         include:
-        - os: ubuntu-latest
-          CPP11: 1
+        - CPP11: 1
           SWIGLANG: ""
-        - os: ubuntu-latest
-          CPP11: 1
+        - CPP11: 1
           SWIGLANG: python
-        - os: ubuntu-latest
-          CPP11: 1
+        - CPP11: 1
           SWIGLANG: python
           PY3: 3
-        - os: ubuntu-latest
-          CPP11: 1
+        - CPP11: 1
           SWIGLANG: tcl
         # let's run all of them, as opposed to aborting when one fails
       fail-fast: false
@@ -50,7 +47,7 @@ jobs:
     - name: ccache
       uses: hendrikmuhs/ccache-action@v1
       with:
-        key: ${{ matrix.os }}-${{ matrix.SWIGLANG }}
+        key: ${{ matrix.os || 'ubuntu-20.04' }}-${{ matrix.SWIGLANG }}
     - name: configure
       shell: bash
       env: