Mk: Wrap FLAVORED 'make clean' in ports_env

[bdrewery]

Summary

This mostly removes the FLAVORS-times multiplier of make variable executions.
For example in security/openssh-portable (3 FLAVORS) it decreases the time
from 17.6s to 6.7s on my system. Much of the cost in that example is
Uses/compiler.mk.

Test Plan

Before:

~/git/ports/main/security/openssh-portable # /usr/bin/time make clean
===>  Cleaning for openssh-portable-8.6.p1,1
===>  Cleaning for openssh-portable-hpn-8.6.p1,1
===>  Cleaning for openssh-portable-gssapi-8.6.p1,1
       17.62 real         8.57 user         8.82 sys

After:

~/git/ports/main/security/openssh-portable # /usr/bin/time make clean
===>  Cleaning for openssh-portable-8.6.p1,1
===>  Cleaning for openssh-portable-hpn-8.6.p1,1
===>  Cleaning for openssh-portable-gssapi-8.6.p1,1
        6.53 real         4.12 user         2.45 sys