This reverts commit a0ef21262f. This
doesn't work in 'nix run' and nix-shell because setns() fails in
multithreaded programs, and Boehm GC mark threads are uncancellable.
Fixes#2646.
This is a little convenience command that opens the Nix expression of
the specified package. For example,
nix edit nixpkgs.perlPackages.Moose
opens <nixpkgs/pkgs/top-level/perl-packages.nix> in $EDITOR (at the
right line number for some editors).
This requires the package to have a meta.position attribute.