Fixed case where a LUKS root is protected by a GPG-encrypted keyfile
and plymouth is in use. Previously, this would use plymouth's
ask-for-password action, passing it a --command argument that
included a pipeline (line 206 in previous version of 00-crypt.sh).
As plymouth does not shell-interpret its command however, this fails.
In this version, we create a temporary script file with the necessary
pipeline command in it, and pass this temporary script to plymouth instead.
The script is in /tmp so disappers when the switch to the real root takes
place.
(In theory, it should be possible to construct a "/bin/sh -c 'cmd'"-style
invocation for the --command, thereby avoiding this, but I have had no
luck with this approach, due to plymouth's argument parser.)