Do no longer use internal rpm data to get the pubkey build time

Do no longer use internal rpm data to get the pubkey build time

Using internal rpm data is just too fragile. This time the code
did not work anymore because the time in the pgpDigParams_s was
changed to a uint32_t, leading to two padding bytes being added.

Rpm's way of calculating the pubkey build time did not change
the last 10 years, so just replicate the way rpm finds the
build time in libsolv. It's simply taking the creation time
of the first signature, which is pretty broken.