tpm2-util: look for tpm2-pcr-signature.json directly in /.extra/

tpm2-util: look for tpm2-pcr-signature.json directly in /.extra/

So far we relied on tmpfiles.d to copy tpm2-pcr-signature.json from
/.extra/ into /run/systemd/. This is racy however if cryptsetup runs too
early, and we cannot unconditionally run it after tmpfiles completed.

hence, let's teach cryptsetup to directly look for the file in /.extra/,
in order to simplify this, and remove the race. But do so only in the
initrd (as only there /.extra/ is a concept).

We generally prefer looking in /run/systemd/, since things are under
user control then. In the regular system we exclusively want that
userspace looks there.

Fixes: #26490