nix-prefetch-url: Add --executable flag

pkgs.fetchurl supports an executable argument, which is especially nice
when downloading a large executable. This patch adds the same option to
nix-prefetch-url.

I have tested this to work on the simple case of prefetching a little
executable:

1. nix-prefetch-url --executable https://my/little/script
2. Paste the hash into a pkgs.fetchurl-based package, script-pkg.nix
3. Delete the output from the store to avoid any misidentified artifacts
4. Realise the package script-pkg.nix
5. Run the executable

I repeated the above while using --name, as well.

I suspect --executable would have no meaningful effect if combined with
--unpack, but I have not tried it.

doc/manual/src/command-ref/nix-prefetch-url.md

@@ -51,6 +51,9 @@ Nix store is also printed.
     result to the Nix store. The resulting hash can be used with
     functions such as Nixpkgs’s `fetchzip` or `fetchFromGitHub`.
 
+  - `--executable`  
+    Set the executable bit on the downloaded file.
+
   - `--name` *name*  
     Override the name of the file in the Nix store. By default, this is
     `hash-basename`, where *basename* is the last component of *url*.