The queue runner used to special-case `localhost` as a remote builder:
Rather than using the normal remote-build (using the
`cmdBuildDerivation` command), it was using the (generally less
efficient, except when running against localhost) `cmdBuildPaths`
command because the latter didn't require a privileged Nix user (so made
testing easier − allowing to run hydra in a container in particular).
However:
1. this means that the build loop can follow two discint code paths depending
on the setup, the irony being that the most commonly used one in production
(the “non-localhost” case) isn't the one used in the testsuite (because all
the tests run against a local store);
2. It turns out that the “localhost” version is buggy in relatively obvious
ways − in particular a failure in a fixed-output derivation or a hash
mismatch isn't reported properly;
3. If the “run in a container” use-case is indeed that important, it can be
(partially) restored using a chroot store (which wouldn't behave excactly
the same way of course, but would be more than good-enough for testing)
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