Make it possible to provide MOZ_PKG_MAC_DSSTORE, MOZ_PKG_MAC_BACKGROUND,
MOZ_PKG_MAC_ICON as environment variables. These files are not where the
build system expects it.

In our build system these files are in
tor-browser-build/projects/browser/Bundle-Data/BaseBrowser.dmg while the
build system expects it to be in browser/branding/tb-*.

For ./mach package to work properly when building for macos we need
these locations. Passing them as env vars is fine for building artifacts
in tbb.

Disable inclusion of browser/installer/windows into packaged development
artifacts. That file is not included by our builds -- at least not by
the ones done by tor-browser-build.

Attempting to zip the unexisting folder will crash `./mach package` when
building inside tor-browser-build.