misc/cgo/testcshared: print a little more about GOROOT on failure

For #13789.

Change-Id: I83973298a35afcf55627f0a72223098306a51f4b
Reviewed-on: https://go-review.googlesource.com/18233Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testcshared/test.bash

@@ -19,6 +19,8 @@ goarch=$(go env GOARCH)
 goroot=$(go env GOROOT)
 if [ ! -d "$goroot" ]; then
 	echo 'misc/cgo/testcshared/test.bash cannnot find GOROOT' 1>&2
+	echo '$GOROOT:' "$GOROOT" 1>&2
+	echo 'go env GOROOT:' "$goroot" 1>&2
 	exit 1
 fi