ci(ada): pass -P modshells.gpr to gprbuild

The Ada (GNAT) workflow ran `gprbuild -j0 -p` with no project file. With
several .gpr/.cgpr files in the tree there is no default project, so
gprbuild exits 7 ("no project file specified and no default project file")
before compiling anything. Specify the project explicitly so the job
actually builds the (now-compiling) Ada sources.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/ada.yml

@@ -27,4 +27,4 @@ jobs:
         sudo apt-get install gnat gprbuild
 
     - name: Build
-      run: gprbuild -j0 -p
+      run: gprbuild -j0 -p -P modshells.gpr