Revert "build: replace which with Bash command built-in"

Revert "build: replace which with Bash command built-in"

This reverts commit c7aec47e5e3a3ff7b5fdaa11cd1e62cae6746acb.

The original commit replaces 'which' with 'command'. Sadly most of
them are not equivalent and for 'which -a', there is no easy
replacements that would not reimplement PATH parsing logic. Hence
revert. Keeping a dependency on which is absolutely fine.

Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>