linux-firmware: fix build when cross-compiling (#349895)

{ stdenvNoCC

, fetchzip

, findutils

, lib

, python3

, rdfind

  # in a perfectly pristine tree, so we can fake just enough of git to run it.

  gitStub = writeShellScriptBin "git" ''

    if [ "$1" == "ls-files" ]; then

      ${lib.getExe findutils} -type f -printf "%P\n"

      find -type f -printf "%P\n"

    else

      echo "Git stub called with unexpected arguments $@" >&2

      exit 1