gitlab-ci: expire the Windows installer artifact after a week.

Currently we run the job on a weekly schedule. If we keep a retention
period of 2 days, it means that people will not have access to a nightly
Windows installer 5 days out of 7, which is not useful.

Gitlab has a retention policy not to delete artifacts for last jobs, but
it apparently actually bases this check on being the last pipeline, not
being actually the last job of a given name. Since we have pipelines
running all the time, this retention policy just won't apply to our
installer job which will get deleted.
Cf. https://gitlab.com/gitlab-org/gitlab/-/issues/332142

.gitlab-ci.yml

@@ -623,7 +623,7 @@ win-installer-nightly:
   artifacts:
     name: "${CI_JOB_NAME}-${CI_COMMIT_REF_SLUG}"
     when: always
-    expire_in: 2 days
+    expire_in: 1 week
     paths:
     - build/windows/installer/_Output
     - installer.log