Set env in exec command (#8917)

# Description

Previously variables with `let-env` were not available after doing an
`exec` command. This PR fixes that

# User-Facing Changes

Can now use environment variables set with nushell after `exec`

# Tests + Formatting

No tests made but formatting has been checked

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->

Co-authored-by: Jelle Besseling <jelle@bigbridge.nl>

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

@@ -548,7 +548,7 @@ impl ExternalCommand {
         }
     }
 
-    fn create_process(
+    pub fn create_process(
         &self,
         input: &PipelineData,
         use_cmd: bool,