Move CI smoke tests onto $OS-latest for all OSs

Fixing this in theory means our builds are more stable, but GitHub
actually rotates through these fairly often, so it also creates churn
and breakage that's unhelpful and generally harder to fix (e.g. right
now, macos-10.15 is in a brownout, so all builds will fail for the next
few hours). Better to follow -latest and at least find out about any
major issues earlier.

Author: pimterry

.github/workflows/ci.yml

@@ -82,13 +82,13 @@ jobs:
       matrix:
         include:
           - platform: Linux
-            os: "ubuntu-20.04"
+            os: "ubuntu-latest"
             test-script: "./unix.sh linux"
-          - platform: Mac 10.15
-            os: "macos-10.15"
+          - platform: Mac
+            os: "macos-latest"
             test-script: "./unix.sh darwin"
           - platform: Windows
-            os: "windows-2019"
+            os: "windows-latest"
             test-script: ".\\windows.bat"
       fail-fast: false
     name: Test on ${{ matrix.platform }}