Lock down environment variable access after all other singletons

To allow other singletons to use new environment variables, move env.go
to the end of the source list, so that its singleton is run last.

Change-Id: Ic92308f72fc0c92a0fedbc5546e337940e32eaab

Android.bp

@@ -94,7 +94,6 @@ bootstrap_go_package {
         "common/config.go",
         "common/defaults.go",
         "common/defs.go",
-        "common/env.go",
         "common/glob.go",
         "common/makevars.go",
         "common/module.go",
@@ -103,6 +102,9 @@ bootstrap_go_package {
         "common/paths.go",
         "common/util.go",
         "common/variable.go",
+
+        // Lock down environment access last
+        "common/env.go",
     ],
     testSrcs: [
         "common/paths_test.go",