nix repl: make symlinks with the :bl command

Requested by ppepino on the Matrix:
https://matrix.to/#/!KqkRjyTEzAGRiZFBYT:nixos.org/$Tb32BS3rVE2BSULAX4sPm0h6CDewX2hClOTGzTC7gwM?via=nixos.org&via=matrix.org&via=nixos.dev

This adds a new command, :bl, which works like :b but also creates
a GC root symlink to the various derivation outputs.

ckie@cookiemonster ~/git/nix -> ./outputs/out/bin/nix repl
Welcome to Nix 2.6.0. Type :? for help.

nix-repl> :l <nixpkgs>
Added 16118 variables.

nix-repl> :b runCommand "hello" {} "echo hi > $out"

This derivation produced the following outputs:
  ./repl-result-out -> /nix/store/kidqq2acdpi05c4a9mlbg2baikmzik44-hello
[1 built, 0.0 MiB DL]
ckie@cookiemonster ~/git/nix -> cat ./repl-result-out
hi

0 files changed, 0 insertions, 0 deletions