[fix] unsplash: fix engine due to anubis bot blocking (#5907)

Unsplash started using [Anubis](https://anubis.techaro.lol/)
for blocking crawlers. Therefore, requests using common
user agents (e.g. Firefox, Chrome) must pass a JavaScript
challenge.

However, other user agents seem unaffected for now, hence
settings the UA to something different does still work.

searx/engines/unsplash.py

@@ -4,6 +4,8 @@
 from urllib.parse import urlencode, urlparse, urlunparse, parse_qsl
 from json import loads
 
+from searx.utils import searxng_useragent
+
 # about
 about = {
     "website": 'https://unsplash.com',
@@ -31,6 +33,13 @@ def clean_url(url):
 def request(query, params):
     params['url'] = search_url + urlencode({'query': query, 'page': params['pageno'], 'per_page': page_size})
     logger.debug("query_url --> %s", params['url'])
+
+    # common user agents (e.g. Firefox, Chrome) are blocked
+    # by Anubis (https://anubis.techaro.lol/)
+    # so we pass the searxng user agent instead, which is not
+    # commonly used by crawlers and hence not blocked
+    params["headers"]["User-Agent"] = searxng_useragent()
+
     return params