Fix error clobbering in kopia cli runner

Error variable was being shared between c.Output and
ioutil.ReadAll. The two could race to set err, and if
Output() finishes first with an error, ReadAll could overwrite
it with nil. Fix is to delegate an independent error for
the pipe read, and fail the test if it is non-nil.
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f