bug#35709: ungoogled-chromium fails to install crx extensions

[swedebugia]

Hi

$ guix describe
Generation 2    May 11 2019 18:46:50    (current)
  guix c087a90
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: c087a90e06d7b9451f802323e24deb1862a21e0f

installed ungoogled-chromium 74.0.3729.131-0.9e33022

downloaded a crx via https://crxextractor.com/
https://chrome.google.com/webstore/detail/pawhash/adgekjfphhgngpdoklolpjenmgneobfg
(Apache 2 licensed)

When dropped into the "extensions"-tab I get an error:
"Package is invalid: CRX_HEADER_INVALID"

same with 
https://chrome.google.com/webstore/detail/ublock-origin/cjpalhdlnbpafiamejdnhcphjbkeiagm 
(gpl-3)

It does not matter whether I enable the "developer-mode"

These work fine on Inox (Chromium 67.0.3396.87 from AUR) on parabola
--
Cheers Swedebugia