cmd/dist: translate /private/var to /var on darwin builders

cmd/dist: translate /private/var to /var on darwin builders

This is ugly but needed on the builders, because they do not set
PWD/GOROOT consistently, and the new content-based staleness
understands that the setting of GOROOT influences the content in
the linker outputs.

Change-Id: I0606f2c70b719619b188864ad3ae1b34432140cb
Reviewed-on: https://go-review.googlesource.com/74070
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>