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

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

Additional context

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

Reproduce the issue in Nushell with the shown let-binding commands, then trace the let binding evaluation and error path to find where the existing binding is removed. Add a regression test that confirms an invalid replacement preserves the previous value, and verify the original reproduction no longer loses $foo.