Revert "Allow NU_LIBS_DIR and friends to be const" (#8501)

Reverts nushell/nushell#8310

In anticipation that we may want to revert this PR. I'm starting the
process because of this issue.

This stopped working
```
let-env NU_LIB_DIRS = [
    ($nu.config-path | path dirname | path join 'scripts')
    'C:\Users\username\source\repos\forks\nu_scripts'
    ($nu.config-path | path dirname)
]
```
You have to do this now instead.
```
const NU_LIB_DIRS = [
    'C:\Users\username\AppData\Roaming\nushell\scripts'
    'C:\Users\username\source\repos\forks\nu_scripts'
    'C:\Users\username\AppData\Roaming\nushell'
]
```

In talking with @kubouch, he was saying that the `let-env` version
should keep working. Hopefully it's a small change.

crates/nu-engine/src/call_ext.rs

@@ -39,7 +39,7 @@ pub trait CallExt {
         &self,
         engine_state: &EngineState,
         stack: &mut Stack,
-        name: &str,
+        pos: usize,
     ) -> Result<T, ShellError>;
 }
 
@@ -111,9 +111,9 @@ impl CallExt for Call {
         &self,
         engine_state: &EngineState,
         stack: &mut Stack,
-        name: &str,
+        pos: usize,
     ) -> Result<T, ShellError> {
-        if let Some(expr) = self.get_parser_info(name) {
+        if let Some(expr) = self.parser_info_nth(pos) {
             let result = eval_expression(engine_state, stack, expr)?;
             FromValue::from_value(&result)
         } else if self.parser_info.is_empty() {