ci: document http proxy bug & disable farms relying on it

All the lava & baremetal farms have to be disabled because of this bug,
and until a fix is found and deployed to each of these farms.

docs/ci/uri-caching.conf

@@ -34,4 +34,6 @@ proxy_set_header Authorization $proxy_authorization;
 
 proxy_pass $proxyuri;
 # Redirect back to ourselves on 301 replies
+# FIXME: this is buggy, it doesn't properly url-encode $1, leading to s3 creds
+# being dropped during the redirection, breaking all the lava & baremetal jobs
 proxy_redirect ~^(.*)$ /cache/?uri=$1;