ci: Add ability to run manually some specific jobs

In some merge requests there are bits (such as memory leaks) that we may want
to test before merging and that the schedules will run them.

As per this add a rule to make them manual, and apply it to some jobs.

.gitlab-ci.yml

@@ -61,6 +61,22 @@ variables:
   except:
     - tags
 
+# Some jobs take a long time and are unlikely to find failures (or will find
+# failures which are not merge-blockers to fix), so they’re executed on a weekly
+# schedule in order to save CI resources and speed up branch pipelines.
+# But for specific merge requests, one may still want to run them, so make
+# possible to run them manually.
+.only-schedules-or-manual:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+      when: never
+    # As per rule before, the following rules will only apply to non MR events
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+      when: always
+    - if: $CI_PIPELINE_SOURCE != "schedule"
+      when: manual
+      allow_failure: true
+
 .build-linux:
   before_script:
     - bash .gitlab-ci/show-execution-environment.sh
@@ -193,7 +209,7 @@ debian-stable-x86_64:
 
 hurd-i386:
   extends:
-    - .only-schedules
+    - .only-schedules-or-manual
   stage: build
   tags:
     - hurd
@@ -224,7 +240,7 @@ hurd-i386:
 muslc-alpine-x86_64:
   extends:
     - .build-linux
-    - .only-schedules
+    - .only-schedules-or-manual
   image: $ALPINE_IMAGE
   stage: build
   needs: []
@@ -255,7 +271,7 @@ muslc-alpine-x86_64:
 installed-tests:
   extends:
     - .build-linux
-    - .only-schedules
+    - .only-schedules-or-manual
   image: $FEDORA_IMAGE
   stage: build
   needs: []
@@ -293,7 +309,7 @@ installed-tests:
 G_DISABLE_ASSERT:
   extends:
     - .build-linux
-    - .only-schedules
+    - .only-schedules-or-manual
   image: $FEDORA_IMAGE
   stage: build
   needs: []
@@ -324,7 +340,7 @@ G_DISABLE_ASSERT:
 valgrind:
   extends:
     - .build-linux
-    - .only-schedules
+    - .only-schedules-or-manual
   image: $FEDORA_IMAGE
   stage: analysis
   needs: []
@@ -432,7 +448,7 @@ msys2-mingw32:
       - _coverage/
 
 msys2-clang64:
-  extends: .only-schedules
+  extends: .only-schedules-or-manual
   stage: build
   tags:
     - win32-ps
@@ -631,7 +647,7 @@ coverage:
 scan-build:
   extends:
     - .build-linux
-    - .only-schedules
+    - .only-schedules-or-manual
   image: $FEDORA_IMAGE
   stage: analysis
   needs: []