fix(core): do not require a config file if required env vars are set

[jawher]

Issue

A regression was introduced in the 2.6.0 release where the CLI will exit with the following error:

scaleway-sdk-go: cannot read config file /root/.config/scw/config.yaml: no such file or directory

Even though all the required env vars SCW_ACCESS_KEY, SCW_SECRET_KEY, SCW_ORGANIZATION_ID, SCW_DEFAULT_ORGANIZATION_ID, SCW_ZONE are set.

The same invocation of the CLI used to work fine with the 2.5.x releases

Proposed Fix

Change the client creation logic to no longer require a configuration file, but instead:

• first load config from env
• if a config file is found, then load it and merge it with the env config

Release note for CHANGELOG:

fix regression where a config file was required even if all required env vars are set

[Codelax]

LGTM
Thanks for your contribution