ci: run gixy to detect nginx config issues

[alxndrsn]

Implemented coincidentally while trying to answer the query at https://github.com/getodk/central/pull/1478/files/30b1cf420e7ea4cd74ed63b4347be6929ab37fef#r2579682481. I was looking for a static analysis tool which might report on paths not covered by existing location directives. No luck there, but this PR came out of it easily.

What has been done to verify that this works as intended?

New test, initially failing, then passing when changes to config are made.

Why is this the best possible solution? Were any other approaches considered?

• gixy-ng seems to be a maintained, popular fork of gixy
  • gixy-ng: https://github.com/dvershinin/gixy/
  • original gixy repo: https://github.com/yandex/gixy/
• currently only considering HIGH-level reports
  • there are some HIGH reports that can be fixed without much effort or controversy
  • there are some MEDIUM reports which are more complicated; this PR ignores those

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No change.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced