CPack/RPM: Fix detection of RPM support for weak dependencies

Add a missing `find_program` call to populate `RPM_EXECUTABLE`.
This was left out of two previous changes that intended to query
the `rpm` executable:

* In commit 9b53eca317 (CPack/RPM: Fix weak dep support, 2021-06-29,
  v3.21.0-rc2~4^2), without `RPM_EXECUTABLE` the query always failed,
  so we were concluding that weak dependencies are not supported even
  when they are.

* In commit 488de6294a (CPack: correctly perform querytags on old
  versions of RPM, 2025-01-09, v4.0.0-rc1~182^2~3), without
  `RPM_EXECUTABLE` the query always failed.  This change has not
  yet been in a release anyway.

Also fix the test case for the "suggests" field to verify this.

Issue: #22350
Reported-by: Balazs Kosaras <balazskosaras@gmail.com>