allow workflow to run on `$CI_JOB_MANUAL` (!211)

this is an attempt at emulating the "_workflow dispatch_" feature of GitHub.

an example of a _manual_ pipeline run: https://gitlab.isae-supaero.fr/dragoon/komodo/-/pipelines/8833

.gitlab-ci.yml

@@ -6,6 +6,7 @@ stages:
 
 workflow:
   rules:
+    - if: $CI_PIPELINE_SOURCE == 'web'
     - if: $CI_COMMIT_MESSAGE =~ /^(draft|no-ci):/
       when: never
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"