Also pass SYSTEMSERVERCLASSPATH to compsvc

Like the existing two, this is a temporary workaround to unblock
development, until we can run derive_classpath in the VM.

Bug: 205750213
Test: can use the variable locally
Change-Id: Ib38a0a1d22fc4528832a1abbb6a26fb9148034c7
diff --git a/compos/composd/src/instance_starter.rs b/compos/composd/src/instance_starter.rs
index 4b3ac1b..3e18c3e 100644
--- a/compos/composd/src/instance_starter.rs
+++ b/compos/composd/src/instance_starter.rs
@@ -152,7 +152,11 @@
 
         // TODO(198211396): Implement correctly.
         service
-            .initializeClasspaths(&env::var("BOOTCLASSPATH")?, &env::var("DEX2OATBOOTCLASSPATH")?)
+            .initializeClasspaths(
+                &env::var("BOOTCLASSPATH")?,
+                &env::var("DEX2OATBOOTCLASSPATH")?,
+                &env::var("SYSTEMSERVERCLASSPATH")?,
+            )
             .context("Initializing *CLASSPATH")?;
         Ok(())
     }