cmd/go: preserve more env vars for TestScript child processes

These are required when testing gccgo.

Change-Id: I6a81d7f4d48292c32a8b3b15ef44d859ab3aa26e
Reviewed-on: https://go-review.googlesource.com/c/163861
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/script_test.go

@@ -83,7 +83,12 @@ type backgroundCmd struct {
 }
 
 var extraEnvKeys = []string{
-	"SYSTEMROOT", // must be preserved on Windows to find DLLs; golang.org/issue/25210
+	"SYSTEMROOT",         // must be preserved on Windows to find DLLs; golang.org/issue/25210
+	"LD_LIBRARY_PATH",    // must be preserved on Unix systems to find shared libraries
+	"CC",                 // don't lose user settings when invoking cgo
+	"GO_TESTING_GOTOOLS", // for gccgo testing
+	"GCCGO",              // for gccgo testing
+	"GCCGOTOOLDIR",       // for gccgo testing
 }
 
 // setup sets up the test execution temporary directory and environment.