A job's purpose is to build an osbuild manifest and upload the results
somewhere. It should not know about which distro was used to generate
the pipeline.
Workers depended on the distro package in two ways:
1. To set an osbuild `--build-env`. This is not necessary anymore in new
versions of osbuild. More importantly, it was wrong: it passed the
runner from the distro that is being built, instead of one that
matches the host.
This patch simply removes that logic.
2. To fetch the output filename with `Distro.FilenameFromType()`. While
that is useful, I don't think it warrants the dependency.
This patch uses the fact that all current pipelines output exactly
one file and uploads that. This should probably be extended in the
future to upload all output files, or to name them explicitly in the
upload target.
The worker should now compile to a smaller binary and do less
unnecessary work on startup (like reading repository files).
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