Expose locals from let expressions to the debugger

[9999years]

Motivation

Closes #8827.

With this file:

let
  r = [];
  x = builtins.throw r;
in
  x

Before:

$ nix eval --file test.nix --debugger
error: cannot coerce a list to a string

nix-repl> :env
Env level 0
builtins true false ...

After:

$ nix eval --file test.nix --debugger
error: cannot coerce a list to a string: [ ]

nix-repl> :env
Env level 0
static: x r

Env level 1
builtins true false ...

nix-repl> r
[ ]

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