Allow installation without forced self-update

Without this commit, Composer cannot be installed *only*. The install action would run self-update if it detects that Composer has been installed previously.

Current behavior has been preserved by running both install and update actions from the default recipe so existing users won't notice any change in behavior if using the default recipe. They will notice different behavior if directly using the install action, so the version should be bumped.

This change also simplifies the code and removes repetition.