ci: AppVeyor: do not install unibilium system-wide (#10464)

Initially added in 685ca180f, but gets built via third-party anyway by now.

ci/build.ps1

@@ -50,7 +50,7 @@ if ($compiler -eq 'MINGW') {
   # in MSYS2, but we cannot build inside the MSYS2 shell.
   $cmakeGenerator = 'Ninja'
   $cmakeGeneratorArgs = '-v'
-  $mingwPackages = @('ninja', 'cmake', 'perl', 'diffutils', 'unibilium').ForEach({
+  $mingwPackages = @('ninja', 'cmake', 'perl', 'diffutils').ForEach({
     "mingw-w64-$arch-$_"
   })