scripts: check-aclrules: use in instead of find

scripts: check-aclrules: use in instead of find

For checking whether a substring is present in a string,
using the pattern:
    "str" in string
is slightly faster than:
    string.find("str") != -1

Use it to shave off 4 % of the runtime of this script that
processes quite a few long source files.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>