Use a specific version of Ubuntu in workflows

Using ubuntu-latest might result in incompatibilities when this image
changes to a different version of Ubuntu. For example, the Ubuntu 24.04
image no longer includes imagemagick, meaning that we'll have to install
it manually when using Ubuntu 24.04.

.github/workflows/db_schema.yml

@@ -12,7 +12,7 @@ permissions:
 
 jobs:
   schema:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     services:
       postgres:
         image: postgres:13.16