fetchurl: support executables

pkgs/build-support/fetchurl/builder.sh

···
 
 finish() {
     set +o noglob
+
+    if [[ $executable == "1" ]]; then
+      chmod +x $downloadedFile
+    fi
+
     runHook postFetch
     stopNest
     exit 0

pkgs/build-support/fetchurl/default.nix

···
   # is communicated to postFetch via $downloadedFile.
   downloadToTemp ? false
 
+, # If true, set executable bit on downloaded file
+  executable ? false
+
 , # If set, don't download the file, but write a list of all possible
   # URLs (resulting from resolving mirror:// URLs) to $out.
   showURLs ? false
···
   outputHash = if outputHash != "" then outputHash else
       if sha256 != "" then sha256 else if sha1 != "" then sha1 else md5;
 
-  outputHashMode = if recursiveHash then "recursive" else "flat";
+  outputHashMode = if (recursiveHash || executable) then "recursive" else "flat";
 
-  inherit curlOpts showURLs mirrorsFile impureEnvVars postFetch downloadToTemp;
+  inherit curlOpts showURLs mirrorsFile impureEnvVars postFetch downloadToTemp executable;
 
   # Doing the download on a remote machine just duplicates network
   # traffic, so don't do that.