In the installer, create the build directory if it doesn't exist.

With the PortAudio dependency managed by vcpkg, it is now possible
to build everything end-to-end, including the installer, from an
empty build directory.

find_visual_studio.ps1

@@ -6,6 +6,8 @@ $InstallationPath = & "$($ProgramFiles)\Microsoft Visual Studio\Installer\vswher
 
 $RedistVersion = @(Get-ChildItem "$($InstallationPath)\VC\Redist\MSVC")[0]
 
+New-Item -ItemType directory -Force build
+
 &{
     Write-Output '// FILE AUTOGENERATED BY find_visual_studio.ps1'
     Write-Output "#define VISUAL_STUDIO_INSTALLATION_PATH ""$($InstallationPath)"""