Add nu lib dirs default (#11248)

# Description

This PR is kind of two PRs in one because they were dependent on each
other.

PR1 -
3de58d4dc2
with update
7fcdb242d9
- This follows our mantra of having everything with defaults written in
nushell rust code. So, that if you run without a config, you get the
same behavior as with the default config/env files. This sets
NU_LIB_DIRS to $nu.config-path/scripts and sets NU_PLUGIN_DIRS to
$nu.config-path/plugins.

PR2 -
0e8ac876fd
- The benchmarks have been broke for some time and we didn't notice it.
This PR fixes that. It's dependent on PR1 because it was throwing errors
because PWD needed to be set to a valid folder and `$nu` did not exist
based on how the benchmark was setup.

I've tested the benchmarks and they run without error now and I've also
launched nushell as `nu -n --no-std-lib` and the env vars exist.

closes #11236

# User-Facing Changes
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# Tests + Formatting
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After Submitting
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src/main.rs

@@ -80,6 +80,33 @@ fn main() -> Result<()> {
     ctrlc_protection(&mut engine_state, &ctrlc);
     sigquit_protection(&mut engine_state);
 
+    // Begin: Default NU_LIB_DIRS, NU_PLUGIN_DIRS
+    // Set default NU_LIB_DIRS and NU_PLUGIN_DIRS here before the env.nu is processed. If
+    // the env.nu file exists, these values will be overwritten, if it does not exist, or
+    // there is an error reading it, these values will be used.
+    let nushell_config_path = if let Some(mut path) = nu_path::config_dir() {
+        path.push("nushell");
+        path
+    } else {
+        // Not really sure what to default this to if nu_path::config_dir() returns None
+        std::path::PathBuf::new()
+    };
+
+    let mut default_nu_lib_dirs_path = nushell_config_path.clone();
+    default_nu_lib_dirs_path.push("scripts");
+    engine_state.add_env_var(
+        "NU_LIB_DIRS".to_string(),
+        Value::test_string(default_nu_lib_dirs_path.to_string_lossy()),
+    );
+
+    let mut default_nu_plugin_dirs_path = nushell_config_path;
+    default_nu_plugin_dirs_path.push("plugins");
+    engine_state.add_env_var(
+        "NU_PLUGIN_DIRS".to_string(),
+        Value::test_string(default_nu_plugin_dirs_path.to_string_lossy()),
+    );
+    // End: Default NU_LIB_DIRS, NU_PLUGIN_DIRS
+
     // This is the real secret sauce to having an in-memory sqlite db. You must
     // start a connection to the memory database in main so it will exist for the
     // lifetime of the program. If it's created with how MEMORY_DB is defined