Sandbox environment variables

So that the build can't access extra information unintentionally.
Particuarly ANDROID_BUILD_TOP is dangerous.

In the future PATH should be locked down as well.

Bug: 307824623
Test: Added a all_genrules target and built that
Change-Id: I88bb0efb0a82529a1c85875a53cf20c8384d07fe
diff --git a/cmd/sbox/sbox.go b/cmd/sbox/sbox.go
index 6459ea1..f3931a4 100644
--- a/cmd/sbox/sbox.go
+++ b/cmd/sbox/sbox.go
@@ -275,7 +275,10 @@
 			if !state.Inherit {
 				return nil, fmt.Errorf("Can't have inherit set to false")
 			}
-			env = append(env, *envVar.Name+"="+os.Getenv(*envVar.Name))
+			val, ok := os.LookupEnv(*envVar.Name)
+			if ok {
+				env = append(env, *envVar.Name+"="+val)
+			}
 		default:
 			return nil, fmt.Errorf("Unhandled state type")
 		}