picard: Fix derivation name.

Name has been changed in c9282c65f43148b6be59c020dc4d72530d5998ff.

Users would probably expect "nix-env -i picard" to work, and as picard
isn't a library it doesn't make sense to set a prefix.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>