Reduce visual clutter in the debugger

[9999years]

Motivation

When you enter the debugger, there's a bunch of visual clutter. This PR reduces it.

Before:

info: breakpoint reached


Starting REPL to allow you to inspect the current state of the evaluator.

Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

nix-repl> :continue
error: uh oh


Starting REPL to allow you to inspect the current state of the evaluator.

Welcome to Nix 2.20.0pre20231222_dirty. Type :? for help.

nix-repl>

After:

info: breakpoint reached

Nix 2.20.0pre20231222_dirty debugger
Type :? for help.
nix-repl> :continue
error: uh oh

nix-repl>

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