More powerful path filters

[hey2022]

Adding the example path filter from the readme to cache.nixos.org results in an error with stage0-posix source bootstrap.

[[upstreams.filters]]
action  = "deny"
field   = "name"
pattern = "*-source"

error: Cannot build '/nix/store/2kbbzfsz1hzd21yj0h989s3z40cgrp58-stage0-posix-1.9.1-source.drv'.
       Reason: builder failed with exit code 1.
       Output paths:
         /nix/store/4f7nm8dblrdy74gmsl62b25rjrh4jcz3-stage0-posix-1.9.1-source
       Last 20 log lines:
       > error: executing '#
       >        #
       >        # Neither your store nor your substituters seems to have:
       >        #
       >        #  /1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9
       >        #
       >        # You can create this path from an already-bootstrapped nixpkgs
       >        # using the following command:
       >        #
       >        #   nix-build '<nixpkgs>' -A make-minimal-bootstrap-sources
       >        #
       >        # Or, if you prefer, you can create this file using only `git`,
       >        # `nix`, and `xz`.  For the commands needed in order to do this,
       >        # see `make-bootstrap-sources.nix`.  Once you have the manual
       >        # result, do:
       >        #
       >        #   nix-store --add-fixed --recursive sha256 ./stage0-posix-1.9.1-source
       >        #
       >        # to add it to your store.
       >        ': No such file or directory

Is there a way I can modify the filter to deny sources with the exception of the stage0-posix source?

I'm not sure if theres a way currently but I was thinking of either making the pattern match with regex or having the filters work in a way kinda like how pam does it with filters applying in an order and filters having stuff like sufficient (skip the rest if a filter matches with allow).