ci/Appveyor: install diffutils via scoop

scoop is a Windows package manager, similar to homebrew on OSX.
It is written on powershell, works in unelevated accounts,
is regularly maintained, and does not need mingw/msys2/cygwin
or WSL environments.
Sample use case is running (busybox) bash,
which can be installed via scoop via "scoop install busybox",
to use CI bash scripts in Appveyor.

Parts of scoop's directory can be cached to avoid downloads.
scoop modifies the registry for persistence environment variables
so scoop itself cannot be cached.

ci/build.ps1

@@ -36,7 +36,8 @@ $scoop = (New-Object System.Net.WebClient).DownloadString('https://get.scoop.sh'
   Invoke-Expression $scoop
 }
 
-scoop install nodejs-lts perl
+scoop install diffutils nodejs-lts perl
+diff3 --version
 node --version
 npm.cmd --version
 perl --version
@@ -76,7 +77,7 @@ if ($compiler -eq 'MINGW') {
   # in MSYS2, but we cannot build inside the MSYS2 shell.
   $cmakeGenerator = 'Ninja'
   $cmakeGeneratorArgs = '-v'
-  $mingwPackages = @('ninja', 'cmake', 'diffutils').ForEach({
+  $mingwPackages = @('ninja', 'cmake').ForEach({
     "mingw-w64-$arch-$_"
   })