test: Don't generate a Nix conf in the makefile

Dependencies

• [2] RNEP7YRS tests: disable sandbox
• [3] FAIJDQKZ
• [4] WHS5RMPO
• [*] G2ZB6464 first test, not yet in buildprocess

Change contents

• edit in t/Makefile.am at line 37
  [3.155]→[3.155:183](∅→∅),[3.183]→[2.0:48](∅→∅)
  − touch nix/etc/nix/nix.conf

  − echo "sandbox = false" >> nix/etc/nix/nix.conf