Mk/bsd.port.mk: Use SETENV for EXTRACT_ENV

Follow the precedent set by the other *_ENV variables in the ports
framework. The only practical effect is a very slight slowdown.

Fixes: 98103f194302 ("Mk/bsd.port.mk: Add EXTRACT_ENV")
Requested by: arrowd
MFH: 2026Q2
Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D57302