Add a flag ‘--check’ to verify build determinism

The flag ‘--check’ to ‘nix-store -r’ or ‘nix-build’ will cause Nix to
redo the build of a derivation whose output paths are already valid.
If the new output differs from the original output, an error is
printed.  This makes it easier to test if a build is deterministic.
(Obviously this cannot catch all sources of non-determinism, but it
catches the most common one, namely the current time.)

For example:

  $ nix-build '<nixpkgs>' -A patchelf
  ...
  $ nix-build '<nixpkgs>' -A patchelf --check
  error: derivation `/nix/store/1ipvxsdnbhl1rw6siz6x92s7sc8nwkkb-patchelf-0.6' may not be deterministic: hash mismatch in output `/nix/store/4pc1dmw5xkwmc6q3gdc9i5nbjl4dkjpp-patchelf-0.6.drv'

The --check build fails if not all outputs are valid.  Thus the first
call to nix-build is necessary to ensure that all outputs are valid.

The current outputs are left untouched: the new outputs are either put
in a chroot or diverted to a different location in the store using
hash rewriting.

scripts/nix-build.in

@@ -121,6 +121,10 @@ for (my $n = 0; $n < scalar @ARGV; $n++) {
         push @instArgs, $arg;
     }
 
+    elsif ($arg eq "--check") {
+        push @buildArgs, $arg;
+    }
+
     elsif ($arg eq "--run-env") { # obsolete
         $runEnv = 1;
     }