buildRustPackage: fix erroneous /bin/sh reference

The fetch-cargo-deps script is written in bash syntax, but it
erroneously ran under the /bin/sh interpreter.

This wasn't noticed because /bin/sh is actually bash in NixOS, but on
some other systems this is not true.

pkgs/build-support/rust/fetch-cargo-deps

@@ -1,5 +1,3 @@
-#! /bin/sh
-
 source $stdenv/setup
 
 set -euo pipefail

pkgs/build-support/rust/fetchcargo.nix

@@ -9,7 +9,7 @@ stdenv.mkDerivation {
   phases = "unpackPhase installPhase";
 
   installPhase = ''
-    ${./fetch-cargo-deps} . "$out"
+    bash ${./fetch-cargo-deps} . "$out"
   '';
 
   outputHashAlgo = "sha256";