Split install.wim before creating USB image

With newer Windows 10 versions, `install.wim` can become larger than
4GiB, which can't be placed in a FAT32 partition anymore. By splitting
it into chunks with `wimsplit` and removing `install.wim`, the larger
images work fine.

wfvm/win.nix

@@ -96,7 +96,7 @@ let
       ''
         #!${pkgs.runtimeShell}
         set -euxo pipefail
-        export PATH=${lib.makeBinPath [ p7zip utils.qemu libguestfs ]}:$PATH
+        export PATH=${lib.makeBinPath [ p7zip utils.qemu libguestfs pkgs.wimlib ]}:$PATH
 
         # Create a bootable "USB" image
         # Booting in USB mode circumvents the "press any key to boot from cdrom" prompt
@@ -106,6 +106,10 @@ let
         mkdir -p win/nix-win
         7z x -y ${windowsIso} -owin
 
+        # Split image so it fits in FAT32 partition
+        wimsplit win/sources/install.wim win/sources/install.swm 3072
+        rm win/sources/install.wim
+
         cp ${autounattend.autounattendXML} win/autounattend.xml
 
         virt-make-fs --partition --type=fat win/ usbimage.img