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.
67723df930