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.
