Environment unduly dropped in interpolated strings

[yannham]

Describe the bug
If one uses variables inside an interpolated expression in the middle of a string, this causes an unexpected "unbound identifier" error. As often with these errors, an environment is probably getting dropped somewhere where it shouldn't.

To Reproduce
Use a variable as an interpolated expression somewhere in the middle of a string:

$nickel <<< '
  let world = "world" in
  "Hello, ${world}!"
'
error: Unbound identifier
  ┌─ <stdin>:3:11
  │
3 │ "Hello, ${world}!"
  │           ^^^^^ this identifier is unbound

Note that the error is not triggered if the variable is alone in the string:

$nickel <<< 'let x = "a" in "${x}"'
Done: Str("a")

Expected behavior
Variables in scope should be accessible in interpolated string. The first example should succeed with result "Hello, world!".