Switch wxMSW 32 bit cross build workflow to Debian Stable

Debian Testing doesn't have 32-bit support any more.

.github/workflows/ci_msw_cross.yml

@@ -68,17 +68,21 @@ jobs:
     # versions of MinGW and Wine and is simpler to test with locally than the
     # bespoke container used by GitHub Actions by default.
     runs-on: ubuntu-latest
-    container: debian:testing-slim
+    container: debian:${{ matrix.debian_release }}-slim
     name: ${{ matrix.name }}
     strategy:
       fail-fast: false
       matrix:
         include:
           - name: wxMSW 64 bits
+            debian_release: testing
             configure_flags: --disable-compat32
           - name: wxMSW 32 bits
+            # Testing doesn't have 32 bit support any more.
+            debian_release: stable
             triplet: i686-w64-mingw32
           - name: wxMSW/Univ
+            debian_release: testing
             configure_flags: --enable-universal --disable-compat32 --disable-debug --disable-optimise
             minimal: true
     env: