Fix security issue in CI (#17545)

The `spack-build-env.txt` file may contains many secrets, but the obvious one is the private signing key in `SPACK_SIGNING_KEY`. This file is nonetheless uploaded as a build artifact to gitlab. For anyone running CI on a public version of Gitlab this is a major security problem. Even for private Gitlab instances it can be very problematic.

Co-authored-by: Scott Wittenburg <scott.wittenburg@kitware.com>