[Backport fc-26.05-dev] webgateway: fix nginx_config sensu check

[platform-pr-manager]

Bot-based backport to fc-26.05-dev, triggered by a label in #2241. Original body:

@flyingcircusio/release-managers

Run it with the correct user and correct config file and extend NixOS
test with possible effects if the check is configured wrong.

PL-135234

Release process

• Created changelog entry using ./changelog.sh

PR release workflow (internal)

• PR has internal ticket
• internal issue ID (PL-…) part of branch name
• internal issue ID mentioned in PR description text
• ticket is on Platform agile board
• if ticket is more urgent than within the next few days, directly contact a member of the Platform team
• set urgency and risk labels
• ensure the merge bot has determined a merge date
• ensure all checks are green
• get a review from a colleague

Design notes

• Provide a feature toggle if the change might need to be adjusted/reverted quickly depending on context. Consider whether the default should be on or off. Example: rate limiting.
• All customer-facing features and (NixOS) options need to be discoverable from documentation. Add or update relevant documentation such that hosted and guided customers can understand it as well.
• Provide warnings in previous platform version and upgrade notes when introducing a breaking change

Security implications

• Security requirements defined? (WHERE)
• Security requirements tested? (EVIDENCE)
  • Tested that file permissions are correct on test vm
  • Tested that check fails when config is invalid
  • Extended automated testing with the effects noticed on the affected VM.