ci: Fix OS/2 building.

Copy and pasted from SDL a little too quickly.  :)

.github/workflows/os2.yml

@@ -10,13 +10,12 @@ jobs:
       - uses: open-watcom/setup-watcom@v0
       - name: Build SDL2
         run: |
+          cd src
           wmake -f Makefile.os2
-      - name: Build tests
-        run: |
-          cd test && wmake -f Makefile.os2
           cd ..
       - name: distclean
         run: |
+          cd src
           wmake -f Makefile.os2 distclean
-          cd test && wmake -f Makefile.os2 distclean
           cd ..
+