fix(ci): checkout repository for get-changed-files

[xudongzheng]

Not sure if/how this was working before without the checkout but all the recent commits that touch app/src seem to fail on get-changed-files.

A few recent examples:

• https://github.com/zmkfirmware/zmk/commit/7be955ff7285a1003455b4d573e843ef713ac584
• https://github.com/zmkfirmware/zmk/commit/a080b5287f9814d90166be523c3cd2969f4d69a3
• https://github.com/zmkfirmware/zmk/commit/2ee76be6fee671042d9740ac0271eb69e2558165

This fixes test failures such as https://github.com/zmkfirmware/zmk/actions/runs/9618851916/job/26533624945.

[xudongzheng]

Glad to see I'm not the only one that noticed this issue.

Ideally your commit with improvements included will get merged soon but I'll keep this open for now in case it gets reviewed more quickly since it's an isolated change.

[petejohanson]

Closing since #2262 is now merged.