TUN-9919: Make RPM postinstall scriplet idempotent

* TUN-9919: Make RPM postinstall scriplet idempotent

Before this commit the postinstall scriptlet isn't idempotent, meaning the users see this error in their upgrade logs:

`ln: failed to create symbolic link '/usr/local/bin/cloudflared': File exists
warning: %post(cloudflared-2025.10.0-1.x86_64) scriptlet failed, exit status 1`

This doesn't break the upgrade (which is why we haven't touched this in 5 years), but adding the -f (force) flag to the symlink command prevents this issue from happening

Closes TUN-9919