REPL internal crash with `let` assignment when the operand is undefined
Using the latest release from https://github.com/arnetheduck/nlvm/actions/runs/7062903531

Command: `echo "let a = x" | ./nlvm r`

Crashes with an internal error after printing the error about the undeclared identifier:

```
stdin(1, 9) Error: undeclared identifier: 'x'
candidates (edit distance, scope distance); see '--spellSuggest': 
 (1, 2): '$' [func declared in /home/xx/Downloads/nlvm-2aa830c/Nim/lib/system/dollars.nim(25, 8)]
 (1, 2): '$' [func declared in /home/xx/Downloads/nlvm-2aa830c/Nim/lib/system/dollars.nim(25, 8)]
 (1, 2): '$' [func declared in /home/xx/Downloads/nlvm-2aa830c/Nim/lib/system/dollars.nim(25, 8)]
 (1, 2): '$' [func declared in /home/xx/Downloads/nlvm-2aa830c/Nim/lib/system/dollars.nim(30, 6)]
stdin(1, 9) Error: expression 'x' has no type (or is ambiguous)
Error: internal error: Unhandled type tyProxy
No stack traceback available
To create a stacktrace, rerun compilation with './koch temp r <file>', see https://nim-lang.github.io/Nim/intern.html#debugging-the-compiler for details
```