Fix the error of androidmk unittest

The unittest checking Android.mk path which is the relative path with
"ANDROID_BUILD_TOP". This can be fix with clean it.

Bug: 319563069
Test: 1. cd build/soong/androidmk/androidmk
      2. go test
Change-Id: Iaf6c331abeadfec3416387f416aaebc0ecfa8a21

androidmk/androidmk/androidmk_test.go

@@ -1730,6 +1730,9 @@ android_test {
 }
 
 func TestEndToEnd(t *testing.T) {
+	// Skip checking Android.mk path with cleaning "ANDROID_BUILD_TOP"
+	t.Setenv("ANDROID_BUILD_TOP", "")
+
 	for i, test := range testCases {
 		expected, err := bpfix.Reformat(test.expected)
 		if err != nil {