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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> **Note**
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crates/nu-utils/src/sample_config/default_env.nu

@@ -8,7 +8,7 @@ def create_left_prompt [] {
     # Perform tilde substitution on dir
     # To determine if the prefix of the path matches the home dir, we split the current path into
     # segments, and compare those with the segments of the home dir. In cases where the current dir
-    # is a parent of the home dir (e.g. `/home`, homedir is `/home/user`), this comparison will 
+    # is a parent of the home dir (e.g. `/home`, homedir is `/home/user`), this comparison will
     # also evaluate to true. Inside the condition, we attempt to str replace `$home` with `~`.
     # Inside the condition, either:
     # 1. The home prefix will be replaced
@@ -86,14 +86,14 @@ $env.ENV_CONVERSIONS = {
 }
 
 # Directories to search for scripts when calling source or use
+# The default for this is $nu.default-config-dir/scripts
 $env.NU_LIB_DIRS = [
-    # FIXME: This default is not implemented in rust code as of 2023-09-06.
     ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
 ]
 
 # Directories to search for plugin binaries when calling register
+# The default for this is $nu.default-config-dir/plugins
 $env.NU_PLUGIN_DIRS = [
-    # FIXME: This default is not implemented in rust code as of 2023-09-06.
     ($nu.default-config-dir | path join 'plugins') # add <nushell-config-dir>/plugins
 ]