ci: Match the GLPK step of main

#2911 made the same fix on `main`, keep the line identical so that merging back does not conflict there.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RobCnYEsXvgBsigxuAwfc3

Author: claude

.github/workflows/custom/after-install/action.yml

@@ -6,7 +6,6 @@ runs:
     - name: Use GLPK on macOS
       if: runner.os == 'macOS'
       run: |
-        # Append: the install action has already put the ccache wrappers and MAKEFLAGS there.
         echo -e 'CPPFLAGS = -I/opt/homebrew/include\nLDFLAGS = -L/opt/homebrew/lib' | tee -a ~/.R/Makevars
       shell: bash