Fix not using system shell on posix (#9449)

* Use system shell + add to bunfig

* Update CMakeLists.txt

* Fix tests + flags

* Use execPath

* windows

* Propagate exit code

* Add test for default shell in use

* Update bun-run-bunfig.test.ts

* Update bun-run-bunfig.test.ts

---------

Co-authored-by: Jarred Sumner <709451+Jarred-Sumner@users.noreply.github.com>

src/shell/shell.zig

@@ -3192,3 +3192,4 @@ pub fn needsEscapeUtf8AsciiLatin1Slow(str: []const u8) bool {
     }
     return false;
 }
+pub const ExitCode = eval.ExitCode;