Ensure macOS vs Linux shasum -a 256 vs sha256sum appropriate usage across all commands

[mokagio]

See https://github.com/Automattic/bash-cache-buildkite-plugin/pull/19/files#r838036148.

Ideally, we should extract the logic that decides which sha* command to call in its own script, so we can refer to it in the other files to avoid duplication.