vpick: make a working copy of the current dname

vpick: make a working copy of the current dname

Since we might edit the string later on by inserting NULs, which then
leads up to using an invalid dname when opening the potential chosen
directory:

[ 4316.957536] testsuite-74.sh[99]: make_choice: entry: mytree_37.0_arm64+2-3
[ 4316.957536] testsuite-74.sh[99]: make_choice: best_version: 37.0
[ 4316.957536] testsuite-74.sh[99]: make_choice: best_filename: mytree_37.0
[ 4316.957536] testsuite-74.sh[99]: Failed to open '//var/lib/machines/mytree.v/mytree_37.0': No such file or directory

Uncovered by vpick tests from TEST-74-AUX-UTILS when run on aarch64.