fix: exporters/cmake/OpenSSLConfig.cmake.in to work for build config

This template file is made to make both:

1. OpenSSLConfig.cmake (CMake config used when building a CMake package
   against an uninstalled OpenSSL build)
2. exporters/OpenSSLConfig.cmake (CMake config that's to be installed
   alongside OpenSSL, and is used when building a CMake package against
   an OpenSSL installation).

Variant 1 was unfortunately getting the internal '_ossl_prefix' variable
wrong, which is due to how the perl snippet builds the command(s) to figure
out its value.  That needed some correction.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24918)