nut: specify pid path during build

nut: specify pid path during build

nut was recently upgraded to 2.8.1 which includes a change in
configure.ac that uses /run for the pidfile if it exists during build.

Explicitly specify --with-pidpath to use the path that was used with
2.8.0 and prior.

The symptom here was that there'd be leftover processes when nut-monitor
was restarted.

Fixes: 82f36e0c781d ("nut: update to 2.8.1")
Fixes: https://github.com/openwrt/packages/issues/24106
Signed-off-by: Matt Merhar <redacted>