ensure world-readability for trusted.gpg in postinst

apt-key creates trusted.gpg if it needs it with 644 nowadays, but before
it ensured this, it was gpg creating it, which gives it by default 600.
Not a problem as long as our gpgv is run as root, but now that we drop
privileges we have to ensure that we can also read trusted.gpg files
created by earlier apt-key versions.

Closes: 647001