fetchtorrent: run hooks when using rqbit

The postFetch and postUnpack hooks ought to be run in fetchtorrent
regardless of the backend in use.  Ensure that's the case, rather than
only running them if the transmission backend is being used.

pkgs/build-support/fetchtorrent/default.nix

@@ -31,7 +31,7 @@ in
   meta ? { },
 }:
 let
-  afterSuccess = writeShellScript "fetch-bittorrent-done.sh" ''
+  transmissionFinishScript = writeShellScript "fetch-bittorrent-done.sh" ''
     ${postUnpack}
     # Flatten the directory, so that only the torrent contents are in $out, not
     # the folder name
@@ -75,17 +75,26 @@ runCommand name
         mkdir -p $downloadedDirectory
         mkdir -p $HOME/.config/transmission
         cp ${jsonConfig} $HOME/.config/transmission/settings.json
+        port="$(shuf -n 1 -i 49152-65535)"
         function handleChild {
           # This detects failures and logs the contents of the transmission fetch
           find $out
           exit 0
         }
         trap handleChild CHLD
-        transmission-cli --port $(shuf -n 1 -i 49152-65535) --portmap --finish ${afterSuccess} --download-dir $downloadedDirectory --config-dir "$HOME"/.config/transmission "$url"
+        transmission-cli \
+            --port "$port" \
+            --portmap \
+            --finish ${transmissionFinishScript} \
+            --download-dir "$downloadedDirectory" \
+            --config-dir "$HOME"/.config/transmission \
+            "$url"
       ''
     else
       ''
         export HOME=$TMP
         rqbit --disable-dht-persistence --http-api-listen-addr "127.0.0.1:$(shuf -n 1 -i 49152-65535)" download -o $out --exit-on-finish "$url"
+        ${postUnpack}
+        ${postFetch}
       ''
   )