Fix certificate validation errors during environment setup on CI

See, for example, https://app.circleci.com/pipelines/github/PrivateStorageio/ZKAPAuthorizer/890/workflows/8e3c1f4b-2c2c-446c-8a37-9e77c5cdd9f6/jobs/2408

cachix: ConnectionError (HttpExceptionRequest Request {
  host                 = "cachix.org"
  port                 = 443
  secure               = True
  requestHeaders       = [("Accept","application/json;charset=utf-8,application/json"),("Authorization","<REDACTED>"),("User-Agent","cachix 0.6.0")]
  path                 = "/api/v1/cache/privatestorage-opensource"
  queryString          = ""
  method               = "GET"
  proxy                = Nothing
  rawBody              = False
  redirectCount        = 10
  responseTimeout      = ResponseTimeoutDefault
  requestVersion       = HTTP/1.1
}
 (InternalException (HandshakeFailed (Error_Protocol ("certificate has unknown CA",True,UnknownCa)))))

Exited with code exit status 1

CircleCI received exit code 1