Issue with Keyring >= 1.2.0 - system password requested - Set keyring 'env' backend by default

[eblondel]

With keyring package version 1.2.0, we get message The 'system' keyring does not exist, enter a keyring password to create it when instantiating the ZenodoManager. This relates to this change r-lib/keyring#95 where file backend is now set by default on Linux OS (env is kept by default on other OS).
For the time being, zen4R will keep using the env backend for storing zenodo user/passwords. This could be refined later depending on needs.