Handle OIDC caching for new keyring entries

[ajkerrigan]

I was testing the new OIDC caching in #628 (yay!) and ran into some errors when I had no existing OIDC entry in my keychain.

First, aws-vault would die immediately with:

aws-vault: error: exec: Failed to get credentials for <my-profile>: The specified item could not be found in the keyring

After a tweak to fix that, it would go through most of the normal credential flow but die later:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x45f99f8]

goroutine 6 [running]:
github.com/99designs/aws-vault/v6/vault.(*SSORoleCredentialsProvider).getRoleCredentials(0xc000314100, 0xc00014c000, 0x0, 0x0)
        github.com/99designs/aws-vault/v6/vault/ssorolecredentialsprovider.go:58 +0xf8
...

After erroring out once, I would see the OIDC entry in my keychain and subsequent runs would work as expected.

This PR is a first pass attempt to address both first-run errors. Happy to talk through it or make additional changes as needed.

[mtibben]

Thanks for the fix @ajkerrigan!

[ajkerrigan]

Thanks for the fix @ajkerrigan!

Thanks for catching/fixing the cache check!