Fixes the output of go env so that variables can be set
more accurately when using Plan 9's rc shell. Specifically,
GOPATH may have multiple components and the current
representation is plain wrong. In practice, we probably
ought to change os. Getenv to produce the right result, but
that requires considerably more thought.

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/66600043