gitlab-ci.yml: set timeout for build to 10h (MR 3337)

A common issue is that building kernels takes longer than the default of
1h, and then MR authors need to adjust the timeout in their pmaports
fork and run it again. By setting this option this is hopefully no
longer the case.

I set it to 10h because that's what we currently have set in the
postmarketOS gitlab configuration and it would allow building MRs for
service packs with multiple kernels.

pmbootstrap has logic built in to detect stale builds - by default it
will stop a build if there was no output for 15 minutes. So having a
large timeout shouldn't be a problem.

.gitlab-ci.yml

@@ -145,6 +145,7 @@ mr-settings:
     expire_in: 1 week
     paths:
       - packages/
+  timeout: 10 h
 
 build-x86_64:
   extends: .build