feat: run one job concurrently to reduce chance of conflicts (#198)

Each run of releaser-pleaser acts on the same global state in the
forge. Therefore, parallel runs are unnecessary.

This commit also communicates to the GitHub and GitLab CI pipelines that
the releaser-pleaser jobs can be cancelled as early as possible.

- On GitHub Actions this can be guaranteed through the workflow
  settings. These settings are copied into each repository that uses
  releaser-pleaser, so users need to update this manually. I will add a
  note to the release notes for this.
- On GitLab CI/CD this requires the user to configure a project level setting to
  "auto-cancel redundant pipelines". We will not recommend user to set
  this, as it is quite invasive and can break their regular CI pipelines.

templates/run.yml

@@ -26,9 +26,19 @@ spec:
 releaser-pleaser:
   stage: $[[ inputs.stage ]]
   needs: $[[ inputs.needs ]]
+
   rules:
     # There is no way to run a pipeline when the MR description is updated :(
     - if: $CI_COMMIT_BRANCH == "$[[ inputs.branch ]]"
+
+  # If a newer releaser-pleaser job runs, this one may be cancelled without problem, releaser-pleaser is idempotent.
+  # This only works if the user enables "auto-cancel redundant pipelines", which we do tell them to, because this is
+  # intrusive and up to the user.
+  interruptible: true
+
+  # No need to have multiple releaser-pleaser jobs running at the same time. They all act on the same global state.
+  resource_group: releaser-pleaser
+
   image:
     name: ghcr.io/apricote/releaser-pleaser:v0.5.1 # x-releaser-pleaser-version
     entrypoint: [ "" ]