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.