Fix --private-key flag for pelican origin token create

[h2zh]

In the PR #2314 that introduced pelican token create command, we changed the function signature of CreateToken from

func (tokenConfig *TokenConfig) CreateToken() (string, error) { ... }

to

func (tokenConfig *TokenConfig) CreateToken(signingKey ...string) (string, error) { ... }

However, we forgot to change the token creation code in the legacy pelican origin token create command, which causes the problem in this issue - it uses the private key in IssuerKeysDirectory to sign the token, instead of the key specified in the command.

Close #2163

[jhiemstrawisc]

I followed the steps in BrianA's reproducer and confirmed this change does generate the token with the correct key. Thanks!