check: Ensure GNU coreutils are used on macOS

[rajveermalviya]

Without this, there are failures when running ./tools/check script with --verbose flag:

Test command: ./tools/check pigeon --all-files --fix --verbose
date: illegal option -- -
usage: date [-jnRu] [-I[date|hours|minutes|seconds]] [-f input_fmt]
            [-r filename|seconds] [-v[+|-]val[y|m|w|d|H|M|S]]
            [[[[mm]dd]HH]MM[[cc]yy][.SS] | new_date] [+output_fmt]
Time now:
zulip-flutter e0df0ed10 • 2025-02-26 05:01:14 +0000

[gnprice]

Good catch, thanks.

Merging with one style tweak:

 
 # shellcheck source=tools/lib/ensure-coreutils.sh
 . "${this_dir}"/lib/ensure-coreutils.sh
+
 # shellcheck source=tools/lib/git.sh
 . "${this_dir}"/lib/git.sh
 

I think I'd be happy to have the . … lines right next to each other if they didn't need those # shellcheck … comments; but since they do, it feels a bit easier to read with a blank line there to visually group that comment with the next line rather than the previous.