build-and-push-assets: more defensive git checkout/pull

[tfrommen]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Be more defensive when attempting to checkout or pull a potentially non-existing remote branch.

• Instead of doing git checkout <BRANCH> || git checkout -b <BRANCH>, which pollutes the workflow log, check if the branch exists (and then check it out), or create it in the CI.
• Only pull a branch that exists (on the remote).

What is the current behavior? (You can also link to an open issue here)

• The Git pull on re-run step fails in case the target branch does not exist yet.
• The logic in the Checkout and merge the built branch and Checkout and merge the release branch steps is not straightforward and will print warnings to the terminal if the target branch does nto exist yet.

What is the new behavior (if this is a feature change)?

• Everything works without warnings or errors/failures.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

Other information:

Closes #140.

[tyrann0us]

Thanks!