Merge pull request #306928 from ramboman/fix-fetchers-proxy-vars

lib.fetchers: Add uppercase proxy environment variables

lib/fetchers.nix

···
     # derivation like fetchurl is allowed to do so since its result is
     # by definition pure.
     "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
+    "HTTP_PROXY" "HTTPS_PROXY" "FTP_PROXY" "ALL_PROXY" "NO_PROXY"
   ];
 
 }