ci: tag linter and sanity check jobs as a "placeholder" jobs

They hardly use any resources and almost finish immediately.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1533>

.gitlab-ci.yml

@@ -52,6 +52,7 @@ trigger:
   stage: 'trigger'
   variables:
     GIT_STRATEGY: none
+  tags: [ 'placeholder-job' ]
   script:
     - echo "Trigger job done, now running the pipeline."
   rules:
@@ -289,6 +290,7 @@ test windows stable:
 rustfmt:
   extends: '.debian:12-stable'
   stage: "lint"
+  tags: [ 'placeholder-job' ]
   needs: []
   script:
     - cargo fmt --version
@@ -297,6 +299,7 @@ rustfmt:
 typos:
   extends: '.debian:12-stable'
   stage: "lint"
+  tags: [ 'placeholder-job' ]
   needs: []
   script:
     - typos
@@ -304,6 +307,7 @@ typos:
 gstwebrtc-api lint:
   image: node:lts
   stage: "lint"
+  tags: [ 'placeholder-job' ]
   needs: []
   script:
     - cd net/webrtc/gstwebrtc-api
@@ -313,6 +317,7 @@ gstwebrtc-api lint:
 check commits:
   extends: '.debian:12-stable'
   stage: "lint"
+  tags: [ 'placeholder-job' ]
   needs: []
   script:
     - ci-fairy check-commits --textwidth 0 --no-signed-off-by