zfs-load-key.sh: ${ZFS} is not the zfs binary

A change[1] was merged yesterday that should refer
to the zfs binary in the initramfs, but is actually
an unset shell variable.

This commit changes this line to call `zfs` directly
like the surrounding code.

[1]: cb5b875

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Garrett Fields <ghfields@gmail.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Ben Cordero <bencord0@condi.me>
Closes #9780

contrib/dracut/90zfs/zfs-load-key.sh.in

@@ -38,7 +38,7 @@ if [ "$(zpool list -H -o feature@encryption $(echo "${BOOTFS}" | awk -F\/ '{prin
     # if the root dataset has encryption enabled
     ENCRYPTIONROOT=$(zfs get -H -o value encryptionroot "${BOOTFS}")
     # where the key is stored (in a file or loaded via prompt)
-    KEYLOCATION=$(${ZFS} get -H -o value keylocation "${ENCRYPTIONROOT}")
+    KEYLOCATION=$(zfs get -H -o value keylocation "${ENCRYPTIONROOT}")
     if ! [ "${ENCRYPTIONROOT}" = "-" ]; then
         KEYSTATUS="$(zfs get -H -o value keystatus "${ENCRYPTIONROOT}")"
         # continue only if the key needs to be loaded