The go command prints paths in errors relative to its current directory.
Since all.bash and run.bash are run in $GOROOT/src, prefer to run
the go command, so that the relative paths are correct.

Before this CL, running all.bash in $GOROOT/src:

	##### Testing race detector
	# net/http
	src/net/http/transport.go:1257: cannot take the address of <node EFACE>

This is wrong (or at least less useful) because there is no $GOROOT/src/src/net/http directory.

Change-Id: I0c0d52c22830d79b3715f51a6329a3d33de52a72
Reviewed-on: https://go-review.googlesource.com/9157Reviewed-by: Rob Pike <r@golang.org>