jenkins: Change node name to "built-in"

This is necessary after the upgrade to the version `2.319.1` [1]

[1] https://www.jenkins.io/doc/book/managing/built-in-node-migration/

Related to T3770

jobs/swh-docker-dev.yaml

 - job:
     name: swh-docker-dev
     description: Build the swh/stack:latest image
-    node: master
+    node: built-in
 
     scm:
       - git:

jobs/tools/docker-clean-images.yaml

 - job:
     name: jenkins-tools/clean-docker
     description: Clean old Docker objects
-    node: master
+    node: built-in
 
     properties:
       - build-discarder:

jobs/tools/dockerfiles.yaml

 - job:
     name: jenkins-tools/swh-jenkins-dockerfiles
-    node: master
+    node: built-in
     scm:
       - git:
           url: http://forge.softwareheritage.org/source/swh-jenkins-dockerfiles.git

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T3770 is now swh/meta#3770 (closed)