Fix Nix detection

[veehaitch]

Detect Nix by probing for the presence of the NIX_STORE environment
variable instead of NIX_PATH. The latter is only set in a nix-shell
session but isn't when building a derivation through nix-build. In
contrast, the NIX_STORE environment variable is set in both cases.

Companion PR to intel/intel-sgx-ssl#111

Signed-off-by: Vincent Haupert mail@vincent-haupert.de

[veehaitch]

Following a discussion in intel/intel-sgx-ssl#111 with @jinghe-INTC, we should merge this PR first.

A short demonstration of $NIX_PATH VS. $NIX_STORE:

Within a nix-shell environment (the way this repository uses Nix):

# nix-shell -p gcc --pure --command "env | grep -E 'NIX_PATH|NIX_STORE'"
NIX_STORE=/nix/store

Within a nix-build (a sandboxed build environment; preferred approach):

# cat default.nix
with import <nixpkgs> { };
runCommand "test-env" { } ''
  env | grep -E 'NIX_PATH|NIX_STORE'
  mkdir $out
''
# nix-build default.nix
NIX_STORE=/nix/store

Please let me know if there's anything I can do to advance this PR, @lzha101. Happy to help!