Pages: fix inheritance of allow_failure from the build job

What does this MR do and why?

While trying to figure out the bug described in #420315 I came across @shinya.maeda's comment in #28393 (comment 215754953)

Based on their suggestion I'm sending this patch, so that the pages:deploy job inherits the state of allow_failure from the pages build job.

Note that setting up all the components needed to be able to test that 1) this change is correct, and 2) this change is enough to fix the bug, is waaay beyond the effort I can put into this. As such, this is a completely blind fix simply based on a GitLaber's comment.

Screenshots or screen recordings

None, see above.

How to set up and validate locally

No clue, see above.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.
  • N/A, see above.