Ease debugging by displaying a portion of the NEXUS_USERNAME variable

[Fgerthoffert]

When provisioning Jahia with artifacts requiring authentication, it can be useful to verify that the necessary environment variables are passed:

• Display only a portion of the username
• To not log the password but assume instead that if username is passed properly, same goes for username

Note: It is likely that in GitHub Actions, the username will be obfuscated entirely anyway, but this will help confirm whether a username is passed (obfuscated data) or none (empty string)