Merge fort-nix/nix-bitcoin#391: examples/shell.nix: Add shell version variable

1f8f2b2139 examples/shell.nix: Add shell version variable (Erik Arvstedt)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK 1f8f2b2139

Tree-SHA512: 30eea3294d7e308d0e3271241e114c65ce9cab75ead91642a5c00b4724d8808bd0221675591856a1d2188570707afeef814ccdce093bea4896031c0ec533403e

examples/shell.nix

@@ -3,6 +3,7 @@ let
 in
   import "${nix-bitcoin}/helper/makeShell.nix" {
     configDir = ./.;
+    shellVersion = "0.0.51";
     # Set this to modify your shell
     # extraShellInitCmds = pkgs: ''<my bash code>'';
   }

helper/makeShell.nix

@@ -1,4 +1,4 @@
-{ configDir, extraShellInitCmds ? (pkgs: "") }:
+{ configDir, shellVersion ? null, extraShellInitCmds ? (pkgs: "") }:
 let
   nixpkgs = (import ../pkgs/nixpkgs-pinned.nix).nixpkgs;
   pkgs = import nixpkgs {};