shell: Disable shellcheck warning in ssh-add-key.sh

Shut up this warning:

> warning[SC2174]: When used with -p, -m only applies to the deepest directory.

This works as intended. We are about to move this script to pkg/lib,
which triggers shellcheck to inspect that file.

pkg/shell/machines/ssh-add-key.sh

@@ -7,6 +7,7 @@ p=${2:-authorized_keys}
 f=$d/$p
 
 if ! test -f "$f"; then
+    # shellcheck disable=SC2174 # yes, we know that -m only applies to the deepest directory
     mkdir -m 700 -p "$d"
     touch "$f"
     chmod 600 "$f"