let binding an existing variable to an invalid object drops the previous binding

[richardwesthaver]

Describe the bug

When an invalid value is let bound to an existing variable an error occurs as expected, but the previous variable binding is dropped.

Note that declaring a mutable binding let mut foo = "good value" and re-assigning that value directly with $foo = bad_value does keep the previous binding after triggering an error

How to reproduce

~> let foo = "valid string"   02/07/2024 11:04:50 PM
~> $foo                       02/07/2024 11:04:57 PM
valid string
~> let foo = invalid_string   02/07/2024 11:05:05 PM
Error: nu::shell::external_command

  × External command failed
   ╭─[entry #6:1:1]
 1 │ let foo = invalid_string
   ·           ───────┬──────
   ·                  ╰── executable was not found
   ╰────
  help: No such file or directory (os error 2)

~> $foo                       02/07/2024 11:05:09 PM
Error: nu::shell::variable_not_found

  × Variable not found
   ╭─[entry #7:1:1]
 1 │ $foo
   · ──┬─
   ·   ╰── variable not found
   ╰────

Expected behavior

The variable foo is still accessible as $foo with a value of "valid string"

Screenshots

No response

Configuration

key	value
version	0.89.1
branch	main
commit_hash	4458aae3d41517d74ce1507ad3e8cd94021feb16
build_os	linux-x86_64
build_target	x86_64-unknown-linux-gnu
rust_version	rustc 1.73.0 (cc66ad468 2023-10-03)
rust_channel	1.73.0-x86_64-unknown-linux-gnu
cargo_version	cargo 1.73.0 (9c4383fb5 2023-08-26)
build_time	2024-01-20 22:17:23 -05:00
build_rust_channel	release
allocator	mimalloc
features	dataframe, default, sqlite, trash, which, zip
installed_plugins	port list

Additional context

https://discord.com/channels/601130461678272522/614593951969574961/1205000221571031050