Restore NU_LIB_DIRS and NU_PLUGIN_DIRS defaults (#10252)

# Description

This PR reverts some changes to NU_LIB_DIRS and NU_PLUGIN_DIRS in the
default_env.config file. Our practice is to have default configs that
match if you were to run `nu -n`. I agree with this goal, but until
someone adds NU_LIB_DIRS and NU_PLUGIN_DIRS, we should revert this
change and not penalize users, breaking their scripts that run with `nu
-c blah` when `blah` is located in the default_env's NU_LIB_DIRS.


# User-Facing Changes
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crates/nu-utils/src/sample_config/default_env.nu

@@ -63,12 +63,14 @@ $env.ENV_CONVERSIONS = {
 
 # Directories to search for scripts when calling source or use
 $env.NU_LIB_DIRS = [
-    # ($nu.default-config-dir | path join 'scripts') # add <nushell-config-dir>/scripts
+    # 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
 $env.NU_PLUGIN_DIRS = [
-    # ($nu.default-config-dir | path join 'plugins') # add <nushell-config-dir>/plugins
+    # 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
 ]
 
 # To add entries to PATH (on Windows you might use Path), you can use the following pattern: