Manually force FLTK 1.3.9 from MSYS2

MSYS2 is unfortunately now only packaging FLTK 1.4.0, which we aren't
compatible with. Manually force the previous version until we can have a
more stable fix in place.

.github/workflows/build.yml

@@ -43,9 +43,13 @@ jobs:
           pacman --sync --noconfirm --needed \
             make mingw-w64-x86_64-toolchain mingw-w64-x86_64-cmake
           pacman --sync --noconfirm --needed \
-            mingw-w64-x86_64-fltk mingw-w64-x86_64-libjpeg-turbo \
+            mingw-w64-x86_64-libjpeg-turbo \
             mingw-w64-x86_64-gnutls mingw-w64-x86_64-pixman \
             mingw-w64-x86_64-nettle mingw-w64-x86_64-gmp
+          # MSYS2 only packages FLTK 1.4 now:
+          # https://github.com/msys2/MINGW-packages/issues/22769
+          pacman --upgrade --noconfirm --needed \
+            https://mirror.msys2.org/mingw/mingw64/mingw-w64-x86_64-fltk-1.3.9-2-any.pkg.tar.zst
       - name: Configure
         run: cmake -G "MSYS Makefiles" -DCMAKE_BUILD_TYPE=Debug -S . -B build
       - name: Build