zed-functions.sh: zed_lock(): don't truncate lock

By appending instead of truncating, we can lock on any file (with write
permissions) instead of only dedicated lock files, since the locking
process itself no longer alters the file in any way

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12042