Fix error display if execve() in the builder fails

After we've send "\2\n" to the parent, we can't send a serialized
exception anymore. It will show up garbled like

  $ nix-build --store /tmp/nix --expr 'derivation { name = "foo"; system = "x86_64-linux"; builder = "/foo/bar"; }'
  this derivation will be built:
    /nix/store/xmdip0z5x1zqpp6gnxld3vqng7zbpapp-foo.drv
  building '/nix/store/xmdip0z5x1zqpp6gnxld3vqng7zbpapp-foo.drv'...

  ErrorErrorEexecuting '/foo/bar': No such file or directory
  error: builder for '/nix/store/xmdip0z5x1zqpp6gnxld3vqng7zbpapp-foo.drv' failed with exit code 1

0 files changed, 0 insertions, 0 deletions