turned out to be a huge performance bottleneck (the text to printed
would always be evaluated, even when it was above the verbosity
level). This reduces fix-ng execution time by over 50%.
gprof(1) is very useful. :-)
writes stdout/stderr of the builder to ${prefix}/var/log/nix/x,
where x is the file name of the derivation expression, e.g.,
/nix/var/log/nix/54256391624be04fcb426048ae3ea0a4-d-pan-0.14.2.nix
Note that consecutive builds of the same expression overwrite,
rather than append to, existing log files.
