refactor REnvVars for substitution in AOT environment

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/REnvVars.java

@@ -161,7 +161,7 @@ public final class REnvVars implements RContext.ContextState {
                  * The only time this can happen legitimately is when run under the graalvm shell,
                  * which does not execute the shell script that normally sets R_HOME.
                  */
-                rHomePath = Paths.get(REnvVars.class.getProtectionDomain().getCodeSource().getLocation().getPath()).getParent();
+                rHomePath = getRHomePath();
             } else {
                 rHomePath = Paths.get(rHome);
             }
@@ -175,6 +175,13 @@ public final class REnvVars implements RContext.ContextState {
         return rHome;
     }
 
+    /**
+     * Separated for substitution in AOT environment.
+     */
+    private static Path getRHomePath() {
+        return Paths.get(REnvVars.class.getProtectionDomain().getCodeSource().getLocation().getPath()).getParent();
+    }
+
     private void checkRHome() {
         String envRHome = envVars.get(R_HOME);
         if (envRHome == null) {