Add git diff

[matheus-o1labs]

Adding feature to CI to make sure git diff returns nothing new after running dune build. There is a dune rule that will generate some ocalm code and it need to ensure that what is pushed is always updated.

Explain your changes:

• adding one-liner code to call 'git diff --exit-code'

Explain how you tested your changes:

• this will be check in the CI