cmd/go: avoid when go.env contain GOTOOLCHAIN=local test fail

Fixes golang#67793

Change-Id: Idb079c18c9f4565d95fbf979c73e578872d3732f

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

@@ -1,5 +1,6 @@
 # Test query for non-defaults in the env
 
+env GOROOT=./a
 env GOTOOLCHAIN=local
 env GOSUMDB=nodefault
 env GOPROXY=nodefault
@@ -44,13 +45,19 @@ go env -changed -json GOARCH
 [GOARCH:amd64] stdout '"GOARCH": "arm64"'
 [!GOARCH:amd64] stdout '"GOARCH": "amd64"'
 
-env GOROOT=./a
 env GOPROXY=s
 go env -changed GOPROXY
 ! stdout 'GOPROXY'
 env GOPROXY=s2
 go env -changed GOPROXY
 stdout 'GOPROXY=''?s2''?'
 
+env GOROOT=./b
+go env -changed
+! stdout 'GOTOOLCHAIN=''?local''?'
+
 --  a/go.env --
 GOPROXY=s
+
+--  b/go.env --
+GOTOOLCHAIN=local