Fix SmartOS build that was broken in 68292290.

SmartOS pretends to be Ubuntu/Debian with respect to its SSL
certificate location.

Change-Id: I5405c6472c8a1e812e472e7301bf6084c17549d6
Reviewed-on: https://go-review.googlesource.com/2704Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>