fix(withFileShareServicePrincipal): add a trailing `/` to the target

[lemeurherve]

This PR adds a trailing / to the target so it's considered as a folder and not a file.

Noticed after https://github.com/jenkins-infra/jenkins.io/pull/7123, from trusted.ci.jenkins.io logs:

Cannot perform sync due to error: trying to sync between different resource types (either file <-> directory or directory <-> file) which is not allowed.sync must happen between source and destination of the same type, e.g. either file <-> file or directory <-> directory.To make sure target is handled as a directory, add a trailing '/' to the target.

Follow-up of:

• 839

• https://github.com/jenkins-infra/jenkins.io/pull/7123

Ref:

• https://github.com/jenkins-infra/helpdesk/issues/3414

[lemeurherve]

Self-merging to restore jenkins.io build on trusted.ci.jenkins.io