GNU mktemp wants 3+ X's in a -t template. The bare `mktemp -t skillcache`
and `mktemp -t skill` calls worked on macOS but crash on Linux before
the script does anything. test-cookbooks.sh fails the same way for all
5 cookbooks because it calls deploy --dry-run.
Adding .XXXXXX is portable: GNU expands the X's, BSD treats them as
prefix. Confirmed on Ubuntu 24.04 / coreutils 9.4: test-cookbooks goes
0/5 -> 5/5.
