nix repl: Scan NixRepl for GC roots
nix repl: Scan NixRepl for GC roots

Fixes #3175.

(cherry picked from commit b244e65cdbc2949af70bd539bf8f3bd2fa952d07)

dtzWill/nix,dtzWill/nix,dtzWill/nix,dtzWill/nix,dtzWill/nix