systemd: use pure debug shell

Instead of referencing the impure /bin/sh, use a proper bash from the store.

pkgs/os-specific/linux/systemd/default.nix

@@ -3,7 +3,7 @@
 , glib, kbd, libxslt, coreutils, libgcrypt, libgpgerror, libidn2, libapparmor
 , audit, lz4, bzip2, libmicrohttpd, pcre2
 , linuxHeaders ? stdenv.cc.libc.linuxHeaders
-, iptables, gnu-efi
+, iptables, gnu-efi, bashInteractive
 , gettext, docbook_xsl, docbook_xml_dtd_42, docbook_xml_dtd_45
 , ninja, meson, python3Packages, glibcLocales
 , patchelf
@@ -64,6 +64,7 @@ stdenv.mkDerivation {
     "-Dloadkeys-path=${kbd}/bin/loadkeys"
     "-Dsetfont-path=${kbd}/bin/setfont"
     "-Dtty-gid=3" # tty in NixOS has gid 3
+    "-Ddebug-shell=${bashInteractive}/bin/bash"
     # while we do not run tests we should also not build them. Removes about 600 targets
     "-Dtests=false"
     "-Dlz4=true"