Ensure branch and tag builds use same cache

What does this MR do?

68513094 had an oversight where while the intent was to ensure both branch and tags jobs use the same cache by appending -branch substring to the tag cache key so it matches what the branch cache would use, the implementation went wrong as the substring ended up being added to the wrong place. This, along with the other mistake of using yaml anchors after defining a key for tag cache (thus overwriting it effectively) resulted in branch cache key being something like Ubuntu-16.04-branch-branch-... (note the double -branch) and tag key being Ubuntu-16.04-branch-....

This MR fixes it.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened