Use derive_classpath in VS for has_classpath_jar

Previously we checked the environment variables directly, but that
doesn't handle an update to derive_classpath that adds new variables.

Bug: 210472252
Test: atest virtualizationservice_device_test
Test: composd_cmd test-compile
Change-Id: I98980b631e534a14a9434745fa64fab308257a1d
diff --git a/virtualizationservice/Android.bp b/virtualizationservice/Android.bp
index 277432b..c3ab39a 100644
--- a/virtualizationservice/Android.bp
+++ b/virtualizationservice/Android.bp
@@ -36,6 +36,7 @@
         "libmicrodroid_metadata",
         "libmicrodroid_payload_config",
         "libonce_cell",
+        "libregex",
         "librustutils",
         "libselinux_bindgen",
         "libserde",