ci: Allow only deploy jobs in branch pipelines from main

ci: Add MESA_CI_POST_MERGE variable

Add the MESA_CI_POST_MERGE variable to inform CI about post-merge jobs. It can be used to disable post-merge jobs based on the below rule:

rules:
  - if: $MESA_CI_POST_MERGE == "1"
    when: never

Signed-off-by: Vignesh Raman vignesh.raman@collabora.com