CI: Fix "nightly" job to provide Python 3.7. EOL, it has been removed

... from recent runner images using Ubuntu 24.

Author: amotl

.github/workflows/nightly.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: ['ubuntu-latest']
+        os: ['ubuntu-22.04']
         python-version: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
         cratedb-version: ['nightly']