external contributions: permission issue with staging area when `arc diff`

As per swh/infra/websites/www.softwareheritage.org-gandi#1519, permission issue exists regarding the staging area.

for that particular instance, the staging area set for the webapp repository is indeed using an ssh://git@forge.softwareheritage.org access.

Possibilities as far as i can see so far:

• Do we use an https url instead? That's what we did for the git repositories of the swh-environment for example.

• Do we use arc's --skip-staging flag as mentioned by the author? Thus updating the wiki page [1] for that.

• Disable the staging area? I'm not sure it's used so far (we talked at some point to use it for the limitations we currently have on ci building stacked diffs IIRC)

• [1] https://wiki.softwareheritage.org/wiki/Code_review_in_Phabricator

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Migrated from T1522 (view on Phabricator)