actions: build: add default settings for storage-repo-*

[cyphar]

The docs state that storage-repo-branch and storage-repo-branch-coverage are optional, but the actual action description doesn't set the defaults which results in errors when trying to operate on storage-repo:

warning: redirecting to https://github.com/cyphar/umoci-fuzz-corpus.git/
fatal: 'origin/' is not a commit and a branch '' cannot be created from it
fatal: '' is not a valid branch name.
2025-05-26 01:54:06,604 - root - ERROR - Could not get coverage: Command '('git', '-C', '/tmp/tmprq4x6jbl', 'checkout', '--orphan', '')' returned non-zero exit status 128..
2025-05-26 01:54:06,605 - root - ERROR - Could not find latest coverage report.

Signed-off-by: Aleksa Sarai [email protected]