Remove android/apex_test.go use of buildDir

This test does not process any bp files it just needs a directory to
pass to TestConfig so it can use that to fake a PathContext. So, this
just replaces buildDir with a call to T.TempDir().

Bug: 182885307
Test: m nothing
Change-Id: I17c0435b7c45e4f8cb8b03a8ec8e57cfe397975b

android/apex_test.go

@@ -121,7 +121,7 @@ func Test_mergeApexVariations(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			config := TestConfig(buildDir, nil, "", nil)
+			config := TestConfig(t.TempDir(), nil, "", nil)
 			ctx := &configErrorWrapper{config: config}
 			gotMerged, gotAliases := mergeApexVariations(ctx, tt.in)
 			if !reflect.DeepEqual(gotMerged, tt.wantMerged) {