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-cli/src/completions/custom_completions.rs

@@ -6,7 +6,7 @@ use nu_protocol::{
     PipelineData, Span, Type, Value,
 };
 use reedline::Suggestion;
-use std::{collections::HashMap, sync::Arc};
+use std::sync::Arc;
 
 use super::completer::map_value_completions;
 
@@ -66,7 +66,7 @@ impl Completer for CustomCompletion {
                 ],
                 redirect_stdout: true,
                 redirect_stderr: true,
-                parser_info: HashMap::new(),
+                parser_info: vec![],
             },
             PipelineData::empty(),
         );