avoid evaluating shell in paths used in apt-key

apt-key creates internally a script (since ~1.1) which it will call to
avoid dealing with an array of different options in the code itself, but
while writing this script it wraps the values in "", which will cause
the shell to evaluate its content upon execution.
To make 'use' of this either set a absolute gpg command or TMPDIR to
something as interesting as:
"/tmp/This is fü\$\$ing cràzy, \$(man man | head -n1 | cut -d' ' -f1)\$!"

If such paths can be encountered in reality is a different question…