property: make cache flushing slight less deterministic

If there is no timer available to seed the stochastic flushing, revert to a
global seed that gets updated each flush.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18906)

(cherry picked from commit 56d4ff6c)