CI: Use an older Ubuntu version

That version has the right system packages we need to run our tests.

Author: ccordoba12

.github/workflows/linux-pip-tests.yml

@@ -17,7 +17,7 @@ concurrency:
 jobs:
   linux:
     name: Linux (pip) - Py${{ matrix.PYTHON_VERSION }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CI: True
       PYTHON_VERSION: ${{ matrix.PYTHON_VERSION }}

.github/workflows/linux-tests.yml

@@ -17,7 +17,7 @@ concurrency:
 jobs:
   linux:
     name: Linux - Py${{ matrix.PYTHON_VERSION }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       CI: True
       PYTHON_VERSION: ${{ matrix.PYTHON_VERSION }}