Fix gz msgs ruby command (#516)

Fixes #462.
Test gz msg command in CI.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>
(cherry picked from commit e3ad7db)

Author: scpeters

.github/ci/after_make.sh

@@ -1,2 +1,6 @@
 # It's necessary to install the python modules for the test.
 make install
+# Test gz command
+export GZ_CONFIG_PATH=/usr/local/share/gz
+export LD_LIBRARY_PATH=/usr/local/lib
+gz msg

core/cmd/cmdmsgs.rb.in

@@ -30,7 +30,7 @@ class Cmd
     unless Pathname.new(exe_name).absolute?
       # We're assuming that the library path is relative to the current
       # location of this script.
-      exe_name = File.expand_path(File.join(File.dirname(__FILE__), LIBRARY_NAME))
+      exe_name = File.expand_path(File.join(File.dirname(__FILE__), exe_name))
     end
 
     # Drop command from list of arguments