release: Adjust where UNAME_r is defined for ports builds

In followup to 80ab50e1de19ca125f05a13937c796d48c4edd4a,
export UNAME_r in Makefile.inc1 instead of Makefile.vm.

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(cherry picked from commit 0be274d3737972532c042d06c53b5807e95aa845)