nixos/jenkins: rework environment handling

Jenkins gets (by default) an additional environment of

  { NIX_REMOTE = "daemon"; }

This has the following problems:

  1. NIX_REMOTE disappears when users specify additional environment
     variables, because defaults have low merge priority.
  2. nix cannot be used without additional NIX_PATH envvar, which is
     currently missing.
  3. If you try to use HTTPS, you'll see that jenkins lacks
     SSL_CERT_FILE envvar, causing it to fail.

This commit adds config.environment.sessionVariables and NIX_REMOTE to
the set of variables that are always there for jenkins, making nix and
HTTPS work out of the box.

services.jenkins.environment is now empty by default.