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-command/src/deprecated/source.rs

@@ -45,7 +45,7 @@ impl Command for Source {
     ) -> Result<PipelineData, ShellError> {
         // Note: this hidden positional is the block_id that corresponded to the 0th position
         // it is put here by the parser
-        let block_id: i64 = call.req_parser_info(engine_state, stack, "block_id")?;
+        let block_id: i64 = call.req_parser_info(engine_state, stack, 0)?;
 
         let block = engine_state.get_block(block_id as usize).clone();
         eval_block_with_early_return(

crates/nu-command/src/env/source_env.rs

@@ -1,8 +1,6 @@
 use std::path::PathBuf;
 
-use nu_engine::{
-    eval_block_with_early_return, find_in_dirs_env, get_dirs_var_from_call, redirect_env, CallExt,
-};
+use nu_engine::{eval_block_with_early_return, find_in_dirs_env, redirect_env, CallExt};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
@@ -44,15 +42,12 @@ impl Command for SourceEnv {
 
         // Note: this hidden positional is the block_id that corresponded to the 0th position
         // it is put here by the parser
-        let block_id: i64 = call.req_parser_info(engine_state, caller_stack, "block_id")?;
+        let block_id: i64 = call.req_parser_info(engine_state, caller_stack, 0)?;
 
         // Set the currently evaluated directory (file-relative PWD)
-        let mut parent = if let Some(path) = find_in_dirs_env(
-            &source_filename.item,
-            engine_state,
-            caller_stack,
-            get_dirs_var_from_call(call),
-        )? {
+        let mut parent = if let Some(path) =
+            find_in_dirs_env(&source_filename.item, engine_state, caller_stack)?
+        {
             PathBuf::from(&path)
         } else {
             return Err(ShellError::FileNotFound(source_filename.span));

crates/nu-command/src/system/nu_check.rs

@@ -1,4 +1,4 @@
-use nu_engine::{find_in_dirs_env, get_dirs_var_from_call, CallExt};
+use nu_engine::{find_in_dirs_env, CallExt};
 use nu_parser::{parse, parse_module_block, unescape_unquote_string};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack, StateWorkingSet};
@@ -106,12 +106,7 @@ impl Command for NuCheck {
             _ => {
                 if let Some(path_str) = path {
                     // look up the path as relative to FILE_PWD or inside NU_LIB_DIRS (same process as source-env)
-                    let path = match find_in_dirs_env(
-                        &path_str.item,
-                        engine_state,
-                        stack,
-                        get_dirs_var_from_call(call),
-                    ) {
+                    let path = match find_in_dirs_env(&path_str.item, engine_state, stack) {
                         Ok(path) => {
                             if let Some(path) = path {
                                 path