curl_args: support :curl user_agent

A small number of casks require the default curl user agent,
`curl/8.7.1`, for requests to work with an upstream server. Though
livecheck doesn't support a `user_agent` parameter yet (this is
forthcoming), I know of a similar number of casks that require a curl
user agent to work as well (there's some overlap). In these cases,
neither `:browser`/`:fake` nor the default Homebrew user agent works.

We've already been using `user_agent: "curl/8.7.1"` for these casks
for some time, so this change simply adds support for  `user_agent:
:curl` to avoid hardcoding a specific version. This works by omitting
the `--user-agent` option when `:curl` is used, so curl will use the
default `curl/8.7.1` user agent. Outside of manual testing to verify
the behavior, I tested this by replacing some `user_agent` arguments
in related casks and checking to make sure they continue to work as
expected.