[DM-41830] Fix URL joining

We ended up with too many slashes after the hostname, which woul confuse pyvo.
This would end up in an unauthorized.

Many ways we can do this, like urljoin, but  I'm just as nervous
about all this parsing going one way or thee others.  Just going
to leave it lay until Frossie wants to do service discovery work.

src/lsst/rsp/utils.py

@@ -64,7 +64,7 @@ def get_service_url(name: str, env_name: Optional[str] = None) -> str:
         return url
 
     fqdn = os.getenv("EXTERNAL_INSTANCE_URL") or ""
-    path = os.getenv(f"{env_name}_ROUTE") or f"/api/{name}"
+    path = os.getenv(f"{env_name}_ROUTE") or f"api/{name}"
     return f"{fqdn}/{path}"