ci: fix date epoch in shell.nix not in Gitlab yml file

This way we fix also local environments for devs with NixOS.
Updates 988c7f65af.

.gitlab-ci.yml

@@ -21,9 +21,6 @@ variables:
   PIPENV_VENV_IN_PROJECT: "true"
   PIPENV_CACHE_DIR: ".cache/"
 
-  # Fix bdist-wheel problem in NixOS by setting source date epoch to a more recent date
-  SOURCE_DATE_EPOCH: 1600000000
-
   # No need for TLS because everything runs on the same machine
   DOCKER_TLS_CERTDIR: ""
 stages:

ci/shell.nix

@@ -44,4 +44,7 @@ stdenv.mkDerivation {
   ];
   LD_LIBRARY_PATH = "${libffi}/lib:${libjpeg.out}/lib:${libusb1}/lib:${libressl.out}/lib";
   NIX_ENFORCE_PURITY = 0;
+
+  # Fix bdist-wheel problem by setting source date epoch to a more recent date
+  SOURCE_DATE_EPOCH = 1600000000;
 }