Autobuild: Make release changelog generation explicit

[hoffie]

Short description of changes

• Autobuild: Use a variable instead of hardcoding the release changelog path twice

• Autobuild: Make release changelog generation explicit
  This was previously done as part of the variable extraction step.
  It's rather surprising to have a script with the name "analyze*" write
  to some Changelog file.

  Therefore, move all of the generation to the same abstraction level (Github workflow).

CHANGELOG: (List along with the other autobuild improvements)

Context: Fixes an issue?
Fixes #2480

Does this change need documentation? What needs to be documented and how?
No.

Status of this Pull Request
Tested on my repo: https://github.com/hoffie/jamulus/runs/6948860313?check_suite_focus=true#step:5:1

What is missing until this pull request can be merged?
Reviews.

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

[hoffie]

Meh, this was supposed to be fixed in #2584 already. Somehow it's missing most of the mentioned commits. I will try to re-submit a fixed up #2584 instead.