install-deps.sh: populate WITH_SEASTAR in the same way as for_make_check

install-deps.sh: populate WITH_SEASTAR in the same way as for_make_check

because `install-deps.sh` is executed using `source`, we have to pass
these options using env variables. but before this change, `WITH_SEASTAR` is used directly,
while `FOR_MAKE_CHECK` is checked and translated to a local variable
`for_make_check`. which, in my opinion, has better readability.

so, in this change, `WITH_SEASTAR` is translated to `with_seastar`
variable in `install-deps.sh`.

Signed-off-by: Kefu Chai <kchai@redhat.com>