rc/env.bash: fix for Go tool-chain build on windows under msys.
rc/env.bash: fix for Go tool-chain build on windows under msys.

The path conversion is done automatically if msys' builtin
shell commands are used.

R=rsc1, peterGo, brainman, Mr_Dark, r
CC=golang-dev
http://codereview.appspot.com/4452042

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old