Use $GIT_PREVIOUS_SUCCESSFUL_COMMIT

[DaSchTour]

I would suggest to change the documentation to use $GIT_PREVIOUS_SUCCESSFUL_COMMIT

For example like this npx nx affected --target=build --base=$GIT_PREVIOUS_SUCCESSFUL_COMMIT as this ensures that everything since the last successful build will be build. This is crucial when pushing multiple commits at once, as this might skip the build for certain apps that were not changed in the last commit.