Add missing dependency on soong.variables

I0dbc5ec58c89b869b59cd0602b82215c4972d799 dropped
configuration.ProductVariablesFileName from the ninja file dependencies.
This causes Soong not to rerun when soong.variables is modified.

Bug: 369141481
Test: m USE_HOST_MUSL=true nothing && m nothing, Soong reruns
Flag; EXEMPT bugfix

Change-Id: Ifb4c580eb72cad2f6555167fe40f43dcefa21d62
diff --git a/cmd/soong_build/main.go b/cmd/soong_build/main.go
index 577c6cc..5b1ae54 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -375,6 +375,7 @@
 	ctx.Register()
 	finalOutputFile, ninjaDeps := runSoongOnlyBuild(ctx)
 
+	ninjaDeps = append(ninjaDeps, configuration.ProductVariablesFileName)
 	ninjaDeps = append(ninjaDeps, usedEnvFile)
 	if shared.IsDebugging() {
 		// Add a non-existent file to the dependencies so that soong_build will rerun when the debugger is