sanity_check: Improve output for manual installation.

We should still display the `source` advice when not in Vagrant or a
Droplet, since that's an important hint for anyone using local
installation on Linux.

We move the "If you are using Vagrant..." text a bit after to
highlight things nicely for folks who are running tools outside
Vagrant.

Also tighten text to avoid line-wrapping on an 80 character console.

tools/lib/sanity_check.py

@@ -8,11 +8,13 @@ def check_venv(filename: str) -> None:
         print(f"You need to run {filename} inside a Zulip dev environment.")
         user_id = os.getuid()
         user_name = pwd.getpwuid(user_id).pw_name
+
+        print(
+            "You can `source /srv/zulip-py3-venv/bin/activate` "
+            "to enter the development environment."
+        )
+
         if user_name != "vagrant" and user_name != "zulipdev":
-            print("If you are using Vagrant, you can `vagrant ssh` to enter the Vagrant guest.")
-        else:
-            print(
-                "You can `source /srv/zulip-py3-venv/bin/activate` "
-                "to enter the Zulip development environment."
-            )
+            print()
+            print("If you are using Vagrant, first run `vagrant ssh` to enter the Vagrant guest.")
         sys.exit(1)