fix(ci): move nightly_only gating from job-level to step-level if

A job-level `if:` can't reference `matrix` at all -- only github/inputs/
needs/vars contexts are available there, confirmed by actionlint and by
this exact commit's own CI run failing outright with "This run likely
failed because of a workflow file issue" (zero jobs registered). Moves
the same condition onto the step that actually runs the tests instead,
where matrix access is already used successfully by the existing db2
install step.

.github/workflows/testcontainers.yml

@@ -42,15 +42,6 @@ concurrency:
 jobs:
   testcontainers:
     runs-on: ubuntu-26.04
-    # Matrix expansion happens before this is evaluated, so `matrix` here is
-    # this specific dialect's own entry, not the whole list -- a dialect
-    # without `nightly_only` set evaluates the left side true (unset is
-    # null, and `null != true` is true) and always runs; one WITH it set
-    # only runs on the cron or a manual dispatch, never on pull_request.
-    if: >-
-      matrix.nightly_only != true ||
-      github.event_name == 'schedule' ||
-      github.event_name == 'workflow_dispatch'
     strategy:
       fail-fast: false
       matrix:
@@ -126,6 +117,16 @@ jobs:
         if: matrix.dialect == 'db2'
         run: uv pip install --system -e .[db2]
       - name: Run testcontainers db_engine_specs tests (${{ matrix.dialect }})
+        # A job-level `if:` can't reference `matrix` (only github/inputs/
+        # needs/vars are available there), so the nightly_only skip has to
+        # live on the step instead. A dialect without `nightly_only` set
+        # evaluates the left side true (unset is null, and `null != true`
+        # is true) and always runs; one WITH it set only runs on the cron
+        # or a manual dispatch, never on pull_request.
+        if: >-
+          matrix.nightly_only != true ||
+          github.event_name == 'schedule' ||
+          github.event_name == 'workflow_dispatch'
         run: |
           pytest --durations-min=2 -v -m testcontainers \
             ./tests/testcontainers/db_engine_specs/test_${{ matrix.dialect }}.py \