Move environment staleness check to soong_ui.

Also delete the now-unnecessary soong_env binary.

Test: Manually. Also checked what happens when a used environment
variable changes.

Change-Id: Ib393e7f444e94819198c6cce4bcd8719d9fd9441
diff --git a/shared/Android.bp b/shared/Android.bp
index 5aa9d54..c79bc2b 100644
--- a/shared/Android.bp
+++ b/shared/Android.bp
@@ -6,6 +6,7 @@
     name: "soong-shared",
     pkgPath: "android/soong/shared",
     srcs: [
+        "env.go",
         "paths.go",
     ],
     deps: [