Activate the nushell virtualenv overlay correctly (#6766)

The activate.nu file works a little differently than its counterparts in
other shells, and it needs to be invoked as an overlay, rather than
sourced directly into the shell.

This fixes an outstanding issue left over from #6323.


Release Notes:

- (Improved) ...
([#6323](https://github.com/zed-industries/zed/issues/6323)).

crates/project/src/terminals.rs

@@ -60,9 +60,11 @@ impl Project {
                 .detach();
 
                 if let Some(python_settings) = &python_settings.as_option() {
+                    let activate_command = Project::get_activate_command(python_settings);
                     let activate_script_path =
                         self.find_activate_script_path(python_settings, working_directory);
                     self.activate_python_virtual_environment(
+                        activate_command,
                         activate_script_path,
                         &terminal_handle,
                         cx,
@@ -104,15 +106,24 @@ impl Project {
         None
     }
 
+    fn get_activate_command(settings: &VenvSettingsContent) -> &'static str {
+        match settings.activate_script {
+            terminal_settings::ActivateScript::Nushell => "overlay use",
+            _ => "source",
+        }
+    }
+
     fn activate_python_virtual_environment(
         &mut self,
+        activate_command: &'static str,
         activate_script: Option<PathBuf>,
         terminal_handle: &Model<Terminal>,
         cx: &mut ModelContext<Project>,
     ) {
         if let Some(activate_script) = activate_script {
             // Paths are not strings so we need to jump through some hoops to format the command without `format!`
-            let mut command = Vec::from("source ".as_bytes());
+            let mut command = Vec::from(activate_command.as_bytes());
+            command.push(b' ');
             command.extend_from_slice(activate_script.as_os_str().as_bytes());
             command.push(b'\n');