Allow config files without credential files and allow overriding paths

[arianvp]

Fixes #948

[arianvp]

cc @flokli

[flokli]

I gave this a try on a system:

• using AWS_CONFIG_FILE pointing to elsewhere
• no ~/.aws/credentials file being present
• and a (legacy) SSO config being present at the location in AWS_CONFIG_FILE.

It worked like a charm! Thanks :heart:

[arianvp]

Please have another look! I added a CHANGELOG

[endgame]

Thank you for your contribution and for your patience. I'll let CI run and look at merging this tomorrow.

[endgame]

CI failures are GHC panics which look like some sort of cache misbehaviour. @brendanhay can you please kick the caches again?