Fix get-image-digest.sh on MacOS
Fix get-image-digest.sh on MacOS

Use sha256sum or openssl depending on which is installed, and correctly
cut the sha from the output.

The output format for sha256sum is `b5bb9d8 path/a/b/c`, and for openssl sha256
it's `SHA256(path/a/b/c) b5bb9d8`, so the cut command needs to pick
different columns depending on the command used.

Signed-off-by: Chance Zibolski <0671e890c734ee0cb7f0027fb3ef2496c29582eb@gmail.com>

cilium/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium