Revert "Revert "Ensure environment dependencies are correct""

This reverts commit 4068a5db6c.

Now the Mac xcode-select and xcrun goes through Config.HostSystemTool,
which will grab $PATH through Config.Getenv

Test: m -j (on mac)
Change-Id: I2632c4fdb2ec961e59944cf02ff165e0fd3c869d

android/env.go

@@ -15,6 +15,9 @@
 package android
 
 import (
+	"os"
+	"strings"
+
 	"android/soong/env"
 
 	"github.com/google/blueprint"
@@ -27,6 +30,19 @@ import (
 // compare the contents of the environment variables, rewriting the file if necessary to cause
 // a manifest regeneration.
 
+var originalEnv map[string]string
+
+func init() {
+	originalEnv = make(map[string]string)
+	for _, env := range os.Environ() {
+		idx := strings.IndexRune(env, '=')
+		if idx != -1 {
+			originalEnv[env[:idx]] = env[idx+1:]
+		}
+	}
+	os.Clearenv()
+}
+
 func EnvSingleton() blueprint.Singleton {
 	return &envSingleton{}
 }