fix $env.FILE_PWD and $env.CURRENT_FILE inside use (#14101)

# Description
Fixes: https://github.com/nushell/nushell/issues/13425

It's just a follow up to #13958.

User input can be a directory, in this case, we need to use the return
value of `find_in_dirs_env` carefully, so in case, I renamed
maybe_file_path to maybe_file_path_or_dir to emphasize it.


# User-Facing Changes
`$env.FILE_PWD` and `$env.CURRENT_FILE` will be more reliable to use.

# Tests + Formatting
Added 2 tests

crates/nu-command/tests/commands/use_.rs

@@ -308,3 +308,47 @@ fn can_use_sub_subname_from_submodule() {
     let actual = nu!(inp);
     assert_eq!(actual.out, "bar")
 }
+
+#[test]
+fn test_use_with_printing_file_pwd() {
+    Playground::setup("use_with_printing_file_pwd", |dirs, nu| {
+        let file = dirs.test().join("mod.nu");
+        nu.with_files(&[FileWithContent(
+            file.as_os_str().to_str().unwrap(),
+            r#"
+                export-env {
+                    print $env.FILE_PWD
+                }
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            "use ."
+        );
+
+        assert_eq!(actual.out, dirs.test().to_string_lossy());
+    });
+}
+
+#[test]
+fn test_use_with_printing_current_file() {
+    Playground::setup("use_with_printing_current_file", |dirs, nu| {
+        let file = dirs.test().join("mod.nu");
+        nu.with_files(&[FileWithContent(
+            file.as_os_str().to_str().unwrap(),
+            r#"
+                export-env {
+                    print $env.CURRENT_FILE
+                }
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            "use ."
+        );
+
+        assert_eq!(actual.out, dirs.test().join("mod.nu").to_string_lossy());
+    });
+}