CI: Try older Ubuntu

Author: effigies

.github/workflows/tests.yml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: ['ubuntu-latest']
+        os: ['ubuntu-18.04']
         python-version: [3.7]
         check: ['test']
         pip-flags: ['']
@@ -35,7 +35,7 @@ jobs:
         nipype-extras: ['doc,tests,profiler']
         optional-depends: ['DEFAULT_OPT_DEPENDS']
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-18.04
             python-version: 3.8
             check: test
             pip-flags: ''