* nix-store, nix-instantiate: added an option `--add-root' to

  immediately add the result as a permanent GC root.  This is the only
  way to prevent a race with the garbage collector.  For instance, the
  old style

    ln -s $(nix-store -r $(nix-instantiate foo.nix)) \
      /nix/var/nix/gcroots/result

  has two time windows in which the garbage collector can interfere
  (by GC'ing the derivation and the output, respectively).  On the
  other hand,

    nix-store --add-root /nix/var/nix/gcroots/result -r \
      $(nix-instantiate --add-root /nix/var/nix/gcroots/drv \
        foo.nix)

  is safe.

* nix-build: use `--add-root' to prevent GC races.

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