kea: make kea environment available to lfc process

kea: make kea environment available to lfc process

If the location of the log lock file is changed using the environment
variable KEA_LOCKFILE_DIR, the kea-dhcp process will work properly, but
the lfc (lease file cleanup) process spawned by it will not inherit this
value and use the default path[1], which will cause the kea server to
print the following message when running:

kea-dhcp4[6767]: Unable to use interprocess sync lockfile (No such file or directory): /var/run/kea/logger_lockfile

Backport a patch to fix this issue[2].

[1] https://gitlab.isc.org/isc-projects/kea/-/issues/3450
[2] https://gitlab.isc.org/isc-projects/kea/-/commit/f477e8ebcc8b8e1f1adaad4d55031084c0ff6f40

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>