Remove the prefix={{branch}}- from the sha tag type which was causing
invalid tag formats like ":-cbc2c2c" when building pull requests.
The {{branch}} placeholder becomes empty for PRs, leaving only the dash
prefix which creates an invalid Docker tag.
Changed from: type=sha,prefix={{branch}}-
Changed to: type=sha
This generates valid tags like "sha-cbc2c2c" for all events.