pbot-vm: fix potential time-out when outputting 0

If the output from a code snippet is `0` without a newline,
Perl treats this as a false value. When Guest::process_command()
returns a false value, that signals to guest-server::serial_server()
that it is the parent returning and so it will not send the result back
to the host.

However, `0\n` is a true value. Solution: always append a final newline to
the guest output.

applets/pbot-vm/guest/lib/Guest.pm

@@ -116,7 +116,7 @@ sub process_command($command, $mod, $user, $tag) {
 
         print STDERR "=" x 40, "\n";
 
-        return $result;
+        return "$result\n";
     } else {
         # wait for child to finish
         waitpid($pid, 0);

lib/PBot/VERSION.pm

@@ -25,8 +25,8 @@ use PBot::Imports;
 # These are set by the /misc/update_version script
 use constant {
     BUILD_NAME     => "PBot",
-    BUILD_REVISION => 4518,
-    BUILD_DATE     => "2022-03-23",
+    BUILD_REVISION => 4520,
+    BUILD_DATE     => "2022-03-31",
 };
 
 sub initialize {}