Remove attrPrefix and run the expected check attrs

[purcell]

When my PR #4 was merged, I noticed that a build broke for my client repo, in which I was setting "attrPrefix". And I realised that either the intention of "attrPrefix" and the "githubActions.checks" attribute are not clear, or there was an error.

Specifically, I was setting attrPrefix to "", and then the generation Actions yml file was building .#${{matrix.attr}}, so it was trying to build .#x86_64-linux.emacs-24-3, for example, and of course that attribute doesn't exist. But what I expected, and what I think you probably planned, was that the githubActions.checks attribute is used both to generate the matrix, and to run those checks later. After all, I can't see why we'd look in one place for checks to run, and then look them up in a different place later — or perhaps I'm missing something.

Anyway, this suggested change makes everything match up: there's no longer an attrPrefix, and the Action steps run .#githubActions.checks.${{matrix.attr}}.

(Related to #4 and #7.)