Fix the bug when getting files changed for pull_request_target event (#26320)

Follow #25229

Copy from
https://github.com/go-gitea/gitea/pull/26290#issuecomment-1663135186

The bug is that we cannot get changed files for the
`pull_request_target` event. This event runs in the context of the base
branch, so we won't get any changes if we call
`GetFilesChangedSinceCommit` with `PullRequest.Base.Ref`.

modules/actions/workflows_test.go

@@ -125,7 +125,7 @@ func TestDetectMatched(t *testing.T) {
 			evts, err := GetEventsFromContent([]byte(tc.yamlOn))
 			assert.NoError(t, err)
 			assert.Len(t, evts, 1)
-			assert.Equal(t, tc.expected, detectMatched(tc.commit, tc.triggedEvent, tc.payload, evts[0]))
+			assert.Equal(t, tc.expected, detectMatched(nil, tc.commit, tc.triggedEvent, tc.payload, evts[0]))
 		})
 	}
 }