Add workaround for mmjstool integrity check failing

[hmhealey]

Copies a workaround for the integrity check issue that we've had people reporting since we upgraded mmjstool

See mattermost/mattermost-plugin-playbooks#868 for more details

Release Note

NONE

[lieut-data]

Thanks, @hmhealey! I ended up patching the first PR in https://github.com/mattermost/mattermost-plugin-playbooks/pull/940/files as we found the preinstall/postinstall hooks weren't reliable enough for use in this context. Would it make sense to include the changes from that PR as well?

[hmhealey]

Yep, I can add those changes to this PR. I forgot how flaky those hooks can be since they used to cause so many problems with mattermost-redux not building before