fetchurl: don't prefer hashed mirrors by default

Right now, when building any FOD that uses `fetchurl` (which is the
majority of ours), `tarballs.nixos.org` will always be contacted
before the actual URL. Given that `tarballs.nixos.org` mainly hosts
the bootstrap tools, which already explicitly set it as the host to pull
from, it doesn't make much sense to force every other `fetchurl`
invocation to first reach out to `tarballs.nixos.org`.

pkgs/build-support/fetchurl/default.nix

@@ -222,7 +222,7 @@ stdenvNoCC.mkDerivation (
 
     # If set, prefer the content-addressable mirrors
     # (http://tarballs.nixos.org) over the original URLs.
-    preferHashedMirrors = true;
+    preferHashedMirrors = false;
 
     # New-style output content requirements.
     inherit (hash_) outputHashAlgo outputHash;