util/mkinstallvars.pl: replace List::Util::pairs with out own

Unfortunately, List::Util::pairs didn't appear in perl core modules
before 5.19.3, and our minimum requirement is 5.10.

Fortunately, we already have a replacement implementation, and can
re-apply it in this script.

Fixes #25366

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25367)

(cherry picked from commit 210dc9a5)