Mk/bsd.port.mk: Add EXTRACT_ENV

Add an EXTRACT_ENV variable which defaults to LC_ALL=C.UTF-8. We set
LC_ALL=C globally to ensure our scripts work the way we intend them to,
but this causes tar and unzip to fail to extract distfiles containing
names that cannot be expressed in ASCII. This was previously masked by
a bug in libarchive which caused tar and unzip to fail to notice that
these names were unconvertible and just pass them through unchanged.

MFH: 2026Q2
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D57295

(cherry picked from commit 98103f19430200b36e9adcb10ed8831ce9ffc42f)