workflows: target *-latest runner images

We are using the *-latest runner images for release builds, but for CI
we're instead using ubuntu-18.04, ubuntu-20.04, and windows-2019
(alongside macos-latest).

Let's run CI against the latest of all three OS images to match what
we're using to build release bits.

Author: mjcheetham

.github/workflows/continuous-integration.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, windows-2019, macos-latest]
+        os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps:
     - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8