Add this to your build.yml file under .github/workflows (if not already there) It happens because some of the Ublue images are big. Do you have this in your build.yml? You could also free some space with an additional step in your build.yml before it writes to /tmp or use a /tmp space that is larger but I’ve never had to do any of that.
maximize_build_space: true
Also, as a side note, Dependabot PRs that try to bump the blue-build/github-action version will often fail because those runs do not have access to the SIGNING_SECRET cosign key, which the action requires via the cosign_private_key input. Because of that, you generally need to update the BlueBuild action version in build.yml manually instead of relying on Dependabot to do it. Example below, you just change the version after the @ sign. You COULD I suppose give the action access to your cosign key, but that’s not something I’d do. 
uses: blue-build/github-action@v1.10.2