.github/workflows/build_wheels.yml: use ubuntu-latest and macos-latest.

This matches other actions, and avoids use of defunct macos-10.15.

Author: julian-smith-artifex-com

.github/workflows/build_wheels.yml

@@ -38,10 +38,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, windows-2019, macos-10.15]
-        #os: [ubuntu-20.04]
-        #os: [windows-2019]
-        #os: [macos-10.15]
+        os: [ubuntu-latest, windows-2019, macos-latest]
 
     steps: