repo add/set-url: add `--no-action`

[rjbou]

to not initialise the repo directly. It will be fetched on next update. See test file

Fixes https://github.com/ocaml/opam/issues/4617

[rjbou]

~TODO: add non initialised information to have a more specific error message~

[dra27]

Note from an dev meeting (a few weeks ago 🙈): could possibly add a flag internally to the repo state so that we can identify the first opam update since the url was set so that the warning could be better (to differentiate, say, the network being down and not warning about all the URLs) i.e. allow opam to know the difference between "this URL has never worked / never been checked" and "this URL has started failing since last time"

[dra27]

Cross-referencing #4617, which this will close!