Fix#6209
When trying to run `nix log <installable>`, try first to resolve the derivation pointed to
by `<installable>` as it is the resolved one that holds the build log.
This has a couple of shortcomings:
1. It’s expensive as it requires re-reading the derivation
2. It’s brittle because if the derivation doesn’t exist anymore or can’t
be resolved (which is the case if any one of its build inputs is missing),
then we can’t access the log anymore
However, I don’t think we can do better (at least not right now).
The alternatives I see are:
1. Copy the build log for the un-resolved derivation. But that means a
lot of duplication
2. Store the results of the resolving in the db. Which might be the best
long-term solution, but leads to a whole new class of potential
issues.
A few notes:
* The `echo hi` is needed to make sure that a file that can be read by
`nix log` is properly created (i.e. some output is needed). This is
known and to be fixed in #6051.
* We explicitly ignore the floating-CA case here: the `$out` of `input3`
depends on `$out` of `input2`. This means that there are actually two
derivations - I assume that this is because at eval time (i.e.
`nix-instantiate -A`) the hash of `input2` isn't known yet and the
other .drv is created as soon as `input2` was built. This is another
issue on its own, so we ignore the case here explicitly.
This seems more correct. It also means one can specify the features a
store should support with --store and remote-store=..., which is useful.
I use this to clean up the build remotes test.
