Regression on OSX: cachix cannot be installed

Example of a successful test (2020-06-03):
https://travis-ci.com/github/doublep/eldev/jobs/343335533

Example of a failed test (2020-06-11):
https://travis-ci.com/github/doublep/eldev/jobs/347726644

Failure seems to start here:

Successful:
524 Please open any issues at https://travis-ci.community/c/languages/nix and cc …
525 nix-env --version 526 nix-env (Nix) 2.0.4 527 nix-instantiate --eval -E ‘with import {}; lib.version or lib.nixpkgsVersion’
528 “20.09pre228165.00df2371122”

Failed:
524 Please open any issues at https://travis-ci.community/c/languages/nix and cc …
525 nix-env --version 526 nix-env (Nix) 2.0.4 527 nix-instantiate --eval -E ‘with import {}; lib.version or lib.nixpkgsVersion’
528 error: evaluation aborted with the following error message: ’
529 This version of Nixpkgs requires Nix >= 2.2, please upgrade:

In the end instead of using binaries through cachix it starts compiling Emacs from source code, which fails (not directly Nix failure, of course). However, even if it works in principle, compiling Emacs would take maybe half an hour, at which point I’d rather give up on testing on OSX completely.

Apparently something changed on Travis. I’m not sure if I should adapt my .travis.yml (and how) or if there should be some centralized update/fix.

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