Allow for overriding publishing conditions

[avaly]

The conditions used now for publishing sizes are tightly coupled to env-ci and the master branch.

See: https://github.com/GoogleChromeLabs/size-plugin/blob/master/src/publish-size.js#L21

It would be nice to support a custom logic there for repositories which use CI that are not recognized by env-ci (e.g. Jenkins) or use a different branch name than master (e.g. main / develop).