CL 123577 added TestScript. The install_rebuild_gopath
test was failing on Plan 9 because it defines a GOPATH
using the ':' separator, while Plan 9 expects the '\000'
separator in environment variables.

This change fixes the script engine by defining a new
":" environment variable set to OS-specific path list
separator.

The install_rebuild_gopath test has been updated to use
"${:}" instead of ":".

Fixes #26421.

Change-Id: I58a97f882cdb48cc0836398b0d98a80ea58041ba
Reviewed-on: https://go-review.googlesource.com/124435
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>