cmd/go: fix TestScript on Plan 9

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>

src/cmd/go/script_test.go

@@ -95,6 +95,7 @@ func (ts *testScript) setup() {
 		"GOROOT=" + testGOROOT,
 		tempEnvName() + "=" + filepath.Join(ts.workdir, "tmp"),
 		"devnull=" + os.DevNull,
+		":=" + string(os.PathListSeparator),
 	}
 
 	if runtime.GOOS == "windows" {

src/cmd/go/testdata/script/README

@@ -149,7 +149,7 @@ and only shows the # comments for earlier phases. For example, here is a
 multi-phase script with a bug in it:
 
 	# GOPATH with p1 in d2, p2 in d2
-	env GOPATH=$WORK/d1:$WORK/d2
+	env GOPATH=$WORK/d1${:}$WORK/d2
 
 	# build & install p1
 	env

src/cmd/go/testdata/script/install_rebuild_gopath.txt

 # GOPATH with p1 in d1, p2 in d2
-[!windows] env GOPATH=$WORK/d1:$WORK/d2
-[windows]  env GOPATH=$WORK/d1;$WORK/d2
+env GOPATH=$WORK/d1${:}$WORK/d2
 
 # build & install p1
 go install -i p1