CI: tags refactoring for [build] stage

Context

Related: #5502 (closed)

This MR refactors the CI configuration to make it easier to set the tags on build jobs. This MR incurs no change in semantics of the configuration: it only shuffles stuff around.

We introduce and apply 2 new templates:

• .tags__template_build: applies to all jobs in the build stage (that do not require arm64)
• .tags__template_build_arm64: applies to all jobs in the build stage that require arm64

Note: This MR should not change the tags of any jobs -- it only makes it easier to switch tags for jobs. In particular, we will use this to change the specifications in executors for the jobs in the build stage.

Manually testing the MR

• In theory nothing changes, so just make sure the CI is running as before.

• You can use the GitLab Swiss knife, with the instructions here https://gitlab.com/arvidnl/glci-swiss-army-knife/#querying-ci-configurations-using-jq to check the value of all tags for a given pipeline type. For more information, ask me (@arvidnl) or refer to the discussion in the comments of this MR.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR