name the application executable when copying to the gamedir

every build artifact also carries Ja2Export.exe, so the glob handed mv
three paths and it failed with "target 'gamedir/ja2.exe' is not a
directory". The assemble job runs on Windows, so the lowercase matrix
name matches the uppercase target file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/release.yml

@@ -85,7 +85,7 @@ jobs:
           set -eux
           for APP in ja2 ja2mapeditor ja2ub ja2ubmapeditor
           do
-            mv artifacts/${APP}/*.exe gamedir/${APP}.exe
+            mv artifacts/${APP}/${APP}.exe gamedir/${APP}.exe
           done
 
       - name: Create version information file