Avoid taking unnecessary ownership of intermediates (#12740)

# Description

Judiciously try to avoid allocations/clone by changing the signature of
functions

- **Don't pass str by value unnecessarily if only read**
- **Don't require a vec in `Sandbox::with_files`**
- **Remove unnecessary string clone**
- **Fixup unnecessary borrow**
- **Use `&str` in shape color instead**
- **Vec -> Slice**
- **Elide string clone**
- **Elide `Path` clone**
- **Take &str to elide clone in tests**

# User-Facing Changes
None

# Tests + Formatting
This touches many tests purely in changing from owned to borrowed/static
data

crates/nu-command/tests/commands/path/exists.rs

@@ -5,7 +5,7 @@ use nu_test_support::playground::Playground;
 #[test]
 fn checks_if_existing_file_exists() {
     Playground::setup("path_exists_1", |dirs, sandbox| {
-        sandbox.with_files(vec![EmptyFile("spam.txt")]);
+        sandbox.with_files(&[EmptyFile("spam.txt")]);
 
         let actual = nu!(
             cwd: dirs.test(),