ci: workaround for nix + gnutls + ubuntu24 issue

[lrstewart]

Release Summary:

Resolved issues:

Description of changes:

@johubertj ran into an interesting issue where he couldn't run our integration tests even using Nix because GnuTLS flagged any priority string containing TLS1.0 or TLS1.1 as invalid. We traced the problem to the "etc/gnutls/config" file on his Ubuntu24 instance, which disabled TLS1.0 and TLS1.1. Nix wasn't overriding the config file when installing or running gnutls, so the system config file was used. See https://www.gnutls.org/manual/html_node/System_002dwide-configuration-of-the-library.html.

We should probably fix this in the upstream Nix repo, but we can work around the problem for now by setting the GNUTLS_SYSTEM_PRIORITY_FILE in our own Nix shell.

Testing:

I created an /etc/gnutls/config file on my EC2 instance:

[overrides]
disabled-version = tls1.0

GnuTLS run from nix develop then report no TLS1.0 support:

[nix openssl-3.0] $ gnutls-cli --list | grep Protocols
Protocols: VERS-TLS1.1, VERS-TLS1.2, VERS-TLS1.3, VERS-DTLS0.9, VERS-DTLS1.0, VERS-DTLS1.2

After applying this fix, even with the config file still in place, GnuTLS started reporting TLS1.0 support again:

[nix openssl-3.0] $ gnutls-cli --list | grep Protocols
Protocols: VERS-TLS1.0, VERS-TLS1.1, VERS-TLS1.2, VERS-TLS1.3, VERS-DTLS0.9, VERS-DTLS1.0, VERS-DTLS1.2

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.