tsh login with --request-id fails when --user differs from active profile username

Expected behavior: When a user already has an active session and assumes an approved access request with tsh login --request-id, passing a --user value that differs from the active session should not break the command.

Current behavior: An error occurs because tsh tries to load non-existent local key files with the --user flag value. Depending on the local ssh-agent state, the failure surfaces in one of two ways:

• Without ssh-agent (or if the agent holds no teleport certs): "ERROR: no SSH auth methods loaded, are you logged in?".
• With ssh-agent running and holding session certs: "ERROR: failed to fetch TLS key for : no credentials: ~/.tsh/keys//.key is empty".

Bug details:

• Teleport version 18.10.x
• Recreation steps
  • Log in to a teleport cluster with tsh login.
  • Create and approve an access request for the logged in user.
  • While logged in, run: tsh login --proxy=<proxy> --user=other-user --request-id=<request-id>.
  • Fails with: "ERROR: failed to fetch TLS key for other-user".
  • Run the same command with SSH_AUTH_SOCK="" or on a system where ssh-agent does not hold teleport certs: SSH_AUTH_SOCK="" tsh login --proxy=<proxy> --user=other-user --request-id=<request-id>.
  • Fails with: "ERROR: no SSH auth methods loaded, are you logged in?".
  • Run the command without --user (tsh login --proxy=<proxy> --request-id=<request-id>) and it succeeds.