ci: fix coverage report paths and tolerate Code Quality not yet enabled

- Write PHPUnit's JUnit and Cobertura reports to reports/ instead of
  var/, since var/ is a Docker volume (Dockerfile's VOLUME /app/var/)
  that isn't bind-mounted to the runner - reports written there never
  reached the host, which is also why the prior junit.xml publish step
  had nothing to read. reports/ is a plain path under the project's
  bind mount, so no extra copy-out step is needed
- Set fail-on-error: false on the coverage upload step: it 404s until
  "Code Quality" is turned on for the repo under Settings > Code
  security > Code quality, a one-time manual step

.gitignore

@@ -1,4 +1,5 @@
 /frankenphp/data
+/reports/
 
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