jbake: fix sandbox build (#46363)

See https://hydra.nixos.org/build/80999410

Without a sandbox the `checkPhase` breaks with the following error:

```
/nix/store/i6vl5lwlz5jbkg4r6p340dwmj6fha3xq-stdenv-linux/setup: bin/jbake: /usr/bin/env: bad interpreter: No such file or directory
jbake did not return correct version
```

Running `patchShebangs` before fixes the issue.

Addresses #45960

pkgs/development/tools/jbake/default.nix

···
 
   buildInputs = [ makeWrapper jre ];
 
+  postPatch = "patchShebangs .";
+
   installPhase = ''
     mkdir -p $out
     cp -vr * $out