File Upload/Download fails in the Web UI with insecure-drop mode set for host user creation

[pschisa]

Expected behavior:
File uploads/downloads will succeed when using the insecure-drop host user creation drop mode to existing directories (like /tmp)

Current behavior:
File upload/downloads fail to existing directories like /tmp when using the insecure-drop mode.

Logs from downloading/uploading to/from an existing /tmp directory

May 10 16:39:11 Test-Cluster-Node2-US-East-Grafana teleport[30587]: 2024-05-10T16:39:11Z DEBU [SUBSYSTEM] starting SFTP process regular/sftp.go:125
May 10 16:39:11 Test-Cluster-Node2-US-East-Grafana teleport[30587]: Could not set shell's cwd to home directory "/home/idontexisthere", defaulting to "/"
May 10 16:39:11 Test-Cluster-Node2-US-East-Grafana teleport[30587]: ERROR: stat /home/idontexisthere: no such file or directory
May 10 16:39:11 Test-Cluster-Node2-US-East-Grafana teleport[30587]: 2024-05-10T16:39:11Z DEBU [SUBSYSTEM] SFTP process finished regular/sftp.go:197

Bug details:

• Teleport version: v15
• Recreation steps: Attempt to use the Web UI file upload/download function to an existing directory after logging in as a non-existent user with insecure-drop mode set

[pschisa]

Seems like this also impacts tsh scp operations as well