Rather than using $<host-TMPDIR>/nix-build-<drvname>-<number>, the
temporary directory is now always /tmp/nix-build-<drvname>-0. This
improves bitwise-exact reproducibility for builds that store $TMPDIR
in their build output. (Of course, those should still be fixed...)

* nix/libstore/build.cc (DerivationGoal)[tmpDirInSandbox]: New field.
(DerivationGoal::startBuilder): Initialize 'useChroot' earlier.  Compute
'tmpDirInSandbox', and use it when populating 'dirsInChroot'.
* doc/guix.texi (Build Environment Setup): Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>