Fix nginx to load css in admin site

[Thovi98]

https://dev.funkwhale.audio/funkwhale/funkwhale/-/issues/2146

Problem

• Description of why you made this PR

https://dev.funkwhale.audio/funkwhale/funkwhale/-/issues/2146 + closes https://github.com/YunoHost-Apps/funkwhale_ynh/issues/230

Solution

• And how do you fix that problem Adapt nginx

PR Status

• [x] Code finished and ready to be reviewed/tested
• [x] The fix/enhancement were manually tested (if applicable)

Automatic tests

Automatic tests can be triggered on https://ci-apps-dev.yunohost.org/ after creating the PR, by commenting "!testme", "!gogogadgetoci" or "By the power of systemd, I invoke The Great App CI to test this Pull Request!". (N.B. : for this to work you need to be a member of the Yunohost-Apps organization)

[Thovi98]

!testme

[yunohost-bot]

Meow :cat2:

[Thovi98]

!testme

[yunohost-bot]

Alrighty!