Limit shellcheck to diff-context for hack repo

[krsna-m]

Shellcheck, which is part of the style action in the actions repo, currently uses the file context which means that it will check for any shell issues across any touched file. Bash/shell is tricky to write and easy to introduce security concerns so this is in place so that it encourages clean up any shell scripts. However, hack has some bloated shell scripts that we will transition away from knative/hack#254. If we use the file context the amount of work needed by engineers is very high to correct the whole file. Therefore the shellcheck style for hack repo should be set to diff-context so that it is more manageable.