rpmkeys does not wait for transaction lock

[pmatilai]

Originally reported at https://bugzilla.redhat.com/show_bug.cgi?id=2413547, a shorter reproducer is

1. install a package as root, pause it mid-transaction
2. try to verify a signature with rpmkeys with a command that has a pipe in front, eg find -name "*.rpm"|rpmkeys -Kv
3. you'll get something like

$ find /tmp/parallel-1.0-1.noarch.rpm |xargs rpmkeys -K
error: can't create transaction lock on /usr/lib/sysimage/rpm/.rpm.lock (Resource temporarily unavailable)
/tmp/parallel-1.0-1.noarch.rpm: digests SIGNATURES NOT OK

...because due to the pipe, rpmkeys does not wait for the transaction lock. If the pipe isn't in place, then the sigcheck waits until the transaction is complete. Which isn't ideal, but much better.