nginx: Fix gixy test host_spoofing

This patch was generated by replacing:
`proxy_set_header Host $http_host` ->
`proxy_set_header Host $host`

Script:
find . -type f -exec sed -i 's|proxy_set_header Host \$http_host|proxy_set_header Host \$host|g' {} \;

Fixes test error:
```
>> Problem: [host_spoofing] The proxied Host header may be spoofed.
Description: In most cases "$host" variable are more appropriate, just use it.
Additional info: https://github.com/yandex/gixy/blob/master/docs/en/plugins/hostspoofing.md
```

`proxy_set_header Host $host` is indeed the recommended default proxy header setting.

production/nginx/location-api-v1-lightning.conf

@@ -5,7 +5,7 @@ location /api/v1/lightning {
 location @mempool-api-v1-lightning {
 	proxy_pass $mempoolMainnetLightning;
 
-	proxy_set_header Host $http_host;
+	proxy_set_header Host $host;
 	proxy_set_header X-Real-IP $remote_addr;
 	proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 	proxy_set_header X-Forwarded-Proto $scheme;