Actually run the Selenium tests in CI

The matrix entries carry env: followed by a plain scalar, so it parses
as the string "DJANGO_SELENIUM_TESTS=true" rather than a mapping, and
matrix.env.DJANGO_SELENIUM_TESTS resolves to nothing. The suite has been
skipping since the job was added.

Make it a mapping. The runner image already provides Firefox and
geckodriver, and the tests run headless when CI is set, so no extra
setup is needed.

Author: novucs

.github/workflows/test.yml

@@ -133,13 +133,13 @@ jobs:
             - python-version: '3.13'
               database: psycopg3
               env:
-                DJANGO_SELENIUM_TESTS=true
+                DJANGO_SELENIUM_TESTS: "true"
             - python-version: '3.14'
               database: postgis3
             - python-version: '3.14'
               database: psycopg3
               env:
-                DJANGO_SELENIUM_TESTS=true
+                DJANGO_SELENIUM_TESTS: "true"
 
     services:
       postgres: