ci: Run PRs on merge result even for i686

This line should have been added in c7f754f.

This mistake caused some i686 builds to fail when the PR was not
rebased, see https://cirrus-ci.com/build/5156197872435200.

.cirrus.yml

@@ -92,6 +92,7 @@ task:
         CC: i686-linux-gnu-gcc
     - env:
         CC: clang --target=i686-pc-linux-gnu -isystem /usr/i686-linux-gnu/include
+  << : *MERGE_BASE
   test_script:
     - ./ci/cirrus.sh
   << : *CAT_LOGS