expose argv[0] as $env.PROCESS_PATH (nushell#11203)

closes nushell#11059 

# Description
I'm not sure what the consensus was after discussing this in discord, so
I'm creating a PR as suggested

# User-Facing Changes
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TBD
# Tests + Formatting
TBD
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crates/nu-cli/src/eval_file.rs

@@ -98,6 +98,10 @@ pub fn evaluate_file(
         "CURRENT_FILE".to_string(),
         Value::string(file_path.to_string_lossy(), Span::unknown()),
     );
+    stack.add_env_var(
+        "PROCESS_PATH".to_string(),
+        Value::string(path, Span::unknown()),
+    );
 
     let source_filename = file_path
         .file_name()