ci: pass MSYS2_ARG_CONV_EXCL inline with make command

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: singaraiona

.github/workflows/build.yml

@@ -69,9 +69,7 @@ jobs:
           RAYFORCE_MAKE_TARGET: lib
 
       - name: Build extensions
-        run: |
-          export MSYS2_ARG_CONV_EXCL='/DEF:'
-          make ext
+        run: MSYS2_ARG_CONV_EXCL='/DEF:' make ext
 
       - name: Package
         run: |