The documentation for the nix: version specifier says:
Note: This option supports all Nix releases, starting with version 1.11.16.
But I attempted today to use Travis to test a Nix build with Nix 2.2 and it failed with the following error (also fails in the same way with Nix 2.3, 2.3.2, 2.3.3 and 2.3.4):
curl: (60) server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
Note that I confirmed it works fine with Nix 2.3.5 and 2.3.7.
I had the same problem in some own computers (xenial and focal) and had to perform an apt update & upgrade to get newer openssl & certificates-ca packages. After that, everything was back to work.
Maybe the travis environments don’t have those packages updated? I’ve tried here rerunning some jobs with xenial, bionic and focal and all them are failing with the same error.
Just a theory, but the date where errors began is a good coincidence.
I have the same issue here with my builds. It’s strange that 4 days passed and it’s still not fixed by Travis team.
UPD: Travis support provided following workaround for .travis.yml file until it’s fixed: