The storeUri variable in the build-remote hook is declared very much to
the start of the main function and a bunch of lines later, the same
variable gets checked via hasPrefix() but it gets assigned *after* that
check when the most suitable machine for the build was choosen.

So I guess this was just a typo in d16fd24
and what we really want is to either checkd the prefix *after* assigning
storeUri or use bestMachine->storeUri directly.

I choose the latter, because the former could introduce even more
regressions if the try block where the variable gets assigned terminates
early.

Nevertheless, the reason why the log output didn't work is because
hasPrefix() checked for "ssh://" in front of storeUri, but if the
storeUri isn't set correctly (or at all), we don't get the log file
descriptor set up properly, leading to no log output.

I've adjusted the remote-builds test to include a regression test for
this, so that we can make sure we get a build output when using remote
builds.

In addition to that I've tested this with two of my build farms and the
build logs are emitted correctly again.

Signed-off-by: aszlig <aszlig@nix.build>

build-remote: Fix missing log output