CI: fix arch linux tests failing to install wxgtk

There hasn't been any such package since the original addition of a
gtk3 version of wxgtk... back in 2017. The "new" wxgtk2 package provided
a virtual provides ever since, so people still depending on "wxgtk"
would get the old gtk2 version. This virtual provides got dropped today,
resulting in the package being uninstallable.

Resolve the provides to its canonical name, thus making it installable
again.

ci/ciimage/arch/install.sh

@@ -8,7 +8,7 @@ pkgs=(
   python python-setuptools python-wheel python-pip python-pytest-xdist python-gobject python-jsonschema
   ninja make git sudo fakeroot autoconf automake patch
   libelf gcc gcc-fortran gcc-objc vala rust bison flex cython go dlang-dmd
-  mono boost qt5-base gtkmm3 gtest gmock protobuf wxgtk gobject-introspection
+  mono boost qt5-base gtkmm3 gtest gmock protobuf wxgtk2 gobject-introspection
   itstool gtk3 java-environment=8 gtk-doc llvm clang sdl2 graphviz
   doxygen vulkan-validation-layers openssh mercurial gtk-sharp-2 qt5-tools
   libwmf valgrind cmake netcdf-fortran openmpi nasm gnustep-base gettext