tools/bc: use more compatible shebang

Update the existing patch to use a shebang that works on systems that
don't have a /bin/bash, e.g. NixOS or GuixSD.

Signed-off-by: Ilya Katsnelson <me@0upti.me>

tools/bc/patches/002-fix-libmath.patch

@@ -10,7 +10,7 @@
 -w
 -q
 -EOS-EOS
-+#! /bin/bash
++#!/usr/bin/env bash
 +sed -e '1   s/^/{"/' \
 +    -e     's/$/",/' \
 +    -e '2,$ s/^/"/'  \