Don't package up the whole Tor Browser in the `mach package` step

When running mach package not only the symlinks are resolved and the omni.ja files compressed but it's attempted to create a final package, be it e.g. a .dmg file for macOS or an .exe file for Windows. We don't want that in the Firefox build step because our bundle is not ready then: we need to add tor, the pluggable transports etc. first and bundle the whole thing up when that's ready.

We achieved this previously by setting INNER_MAKE_PACKAGE=true when running the packaging step but that does not seem to be doable anymore with the switch to mach.

We should write a patch, if necessary, to get the old behavior back. Meanwhile, for macOS, we ship a workaround (see legacy/trac#24632 (moved)) as the build breaks without it. Windows might be affected here, too (and even on Linux a final .bz2 file is created which is not necessary for our purposes).