configure: Set the right $PATH for Nix.

Dependencies

• [2] XVBNSBNG configure: Make sure we get a recent `nix-store'.
• [3] FV2M6MOT hydra: use autoconf/-make
• [4] MBEEIXWJ configure: Call the variable $NIX_STORE_PROGRAM, not $NIX_STORE.

Change contents

• replacement in configure.ac at line 41
  [2.1]→[2.1:19](∅→∅)
  − PATH="$nix:$PATH"
  [2.1]

  [2.19]
  + PATH="$nix/bin:$PATH"