ci : add fetch-depth to xcframework upload (#12195)

This commit adds the fetch-depth: 0 option to the checkout action in the
build.yml workflow file (0 meaning that it fetches the complete
history). The default value is 1 when not specified which only fetches
the latest commit.

This is necessary to ensure that `git rev-list --count HEAD` counts the
total number of commits in the history. Currently because the default is
being used the name of the xcframework artifact is always
llama-b1-xcframework.