ci: rename misleading *-postmerge stages to *-nightly

These stages are for the jobs that are skipped in merge pipelines,
automatically run in nightly pipelines, and are available to run
manually in other pipelines.

None of these ever run in post-merge pipelines.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34590>

.gitlab-ci.yml

@@ -169,23 +169,23 @@ stages:
   - build-only
   - code-validation
   - amd
-  - amd-postmerge
+  - amd-nightly
   - intel
-  - intel-postmerge
+  - intel-nightly
   - nouveau
-  - nouveau-postmerge
+  - nouveau-nightly
   - arm
-  - arm-postmerge
+  - arm-nightly
   - broadcom
-  - broadcom-postmerge
+  - broadcom-nightly
   - freedreno
-  - freedreno-postmerge
+  - freedreno-nightly
   - etnaviv
-  - etnaviv-postmerge
+  - etnaviv-nightly
   - software-renderer
-  - software-renderer-postmerge
+  - software-renderer-nightly
   - layered-backends
-  - layered-backends-postmerge
+  - layered-backends-nightly
   - performance
   - deploy