cli/config: some minor cleanups

[thaJeztah]

I have some more changes in this area, but need to dust it off; let's open these already as a PR 😅

cli/config/configfile: inline getConfiguredCredentialStore

It was a premature abstraction; the "nil" check for the map was redundant,
making it literally a 1-liner, so just inline it.

cli/config/configfile: use more idiomatic receiver name

Use a shorter name, which is more idiomatic, and prevents accidental
shadowing of types or arguments.

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✅ All modified and coverable lines are covered by tests.

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[thaJeztah]

Hm... I messed up something;

#18 64.05 === FAIL: cli/config/configfile TestGetAllCredentialsCredsStore (0.00s)
#18 64.05     file_test.go:238: assertion failed: 
#18 64.05         --- expected
#18 64.05         +++ authConfigs
#18 64.05           map[string]types.AuthConfig{
#18 64.05         - 	"example.com": {Username: "user", Password: "pass"},
#18 64.05           }
#18 64.05         
#18 64.05     file_test.go:239: assertion failed: 1 (int) != 0 (testCredsStore.(*mockNativeStore).GetAllCallCount int)
#18 64.05 
#18 64.05 === FAIL: cli/config/configfile TestGetAllCredentialsCredStoreAndCredHelper (0.00s)
#18 64.05     file_test.go:402: assertion failed: 
#18 64.05         --- expected
#18 64.05         +++ authConfigs
#18 64.05           map[string]types.AuthConfig{
#18 64.05           	"credhelper.com": {Username: "cred_helper_user", Password: "cred_helper_pass"},
#18 64.05         - 	"credstore.com":  {Username: "cred_store_user", Password: "cred_store_pass"},
#18 64.05           }
#18 64.05