erlang: Change order of commands in `postPach` and `postInstall`.

Erlang's generic-builder isn't using the right /bin/sh in some
situations, causing build errors either in the erlang build itself or
in derivations depending on it.

This patch runs `patchShebangs make/` before running the rest of the
`postPatch` phase, hopefully using the right shell early enough.

This might be a solution for #36853, #36823 and #37638.