git: Use environment from login shell to search for system git binary, and prefer it to the bundled binary (#39302)

Closes #38571

Release Notes:

- git: Fixed git features not working when git was installed in an
unusual location.

---------

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>

crates/project/src/project_tests.rs

@@ -7797,7 +7797,9 @@ async fn test_staging_random_hunks(
         path!("/dir/.git").as_ref(),
         &[("file.txt", index_text.clone())],
     );
-    let repo = fs.open_repo(path!("/dir/.git").as_ref()).unwrap();
+    let repo = fs
+        .open_repo(path!("/dir/.git").as_ref(), Some("git".as_ref()))
+        .unwrap();
 
     let project = Project::test(fs.clone(), [path!("/dir").as_ref()], cx).await;
     let buffer = project