Revert "Try Ubuntu 20.04 and see if those tex packages work better."

This reverts commit c41a3a2.

.github/workflows/cmake.yml

@@ -12,7 +12,7 @@ jobs:
     # well on Windows or Mac.  You can convert this to a matrix build if you need
     # cross-platform coverage.
     # See: https://docs.github.com/en/free-pro-team@latest/actions/learn-github-actions/managing-complex-workflows#using-a-build-matrix
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@v2