builder: fix windows environment
builder: fix windows environment

Not causing problems yet, but no need to
keep using cygwin paths now that we're off cygwin.

R=alex.brainman
CC=golang-dev
http://codereview.appspot.com/5656053

dsriram/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,tv42/old-go,tv42/old-go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,dsriram/golang