stdenv: run patchShebangs on the configure script when it's a file

if the configure script has a `/usr/bin/env` or some other shebang which
is not in the sandbox then there will be errors such as

`...-stdenv-linux/setup: line 1299: ./configure: cannot execute: required file not found`

There are 250 files which `patchShebangs` `./configure`

https://github.com/search?q=NOT+is%3Afork+lang%3Anix+%2FpatchShebangs+.%5C%2Fconfigure%2F&type=code

pkgs/stdenv/generic/setup.sh

@@ -1273,6 +1273,10 @@ configurePhase() {
                 prependToVar configureFlags --disable-static
             fi
         fi
+
+        if [ -z "${dontPatchShebangsInConfigure:-}" ]; then
+            patchShebangs --build "$configureScript"
+        fi
     fi
 
     if [ -n "$configureScript" ]; then