Add the .INSTALL script needed for kmsgrab to work

This also removes the standalone PKGBUILD artifact because
our PKGBUILD has external dependencies now.

packaging/linux/Arch/PKGBUILD

@@ -8,6 +8,7 @@ pkgdesc="@PROJECT_DESCRIPTION@"
 arch=('x86_64' 'aarch64')
 url=@PROJECT_HOMEPAGE_URL@
 license=('GPL3')
+install=sunshine.install
 
 depends=('avahi'
          'boost-libs'