Fix 'on: push' branch and only log into Conan remote when pushing

[bthomee]

• The on: push used the main branch, but since this is a fork of the conan center index repo, we need to use the master branch instead.
• To make it possible for forks of the rippled repo to pull recipes from the Conan registry, the remote name and URL were moved from org/repo variables into env vars. Here the same is done.
• We now only add the remote and dry-run the upload in the XRPLF repo, and only log into the remote and do the upload when pushing.

[mathbunnyru]

Let's remove some ifs and also hash-pin actions