Log Message:
go: Set GOTMPDIR.

Go uses TMPDIR for ephemeral objects, but unlike other compilers does not
clean up quickly enough, so even with a reasonably large tmpfs limit of 1G
some package builds can fail due to too many leftover objects.

Use the same directory as GOCACHE and ensure it is created after distfile
extraction.

Tested in a bulk build on SmartOS and confirmed to fix a number of package
builds.  No feedback on proposal to tech-pkg@.