[Fleet]: when upgrading an agent, it fails because of existing simlink elastic-agent.exe.prev

[merlixelastic]

Kibana version: 7.12.1

Host OS: Windows 10 x64

Steps to reproduce:
Try an upgrade Elastic Agent from 7.12.0 to 7.12.1 (triggered from Kibana). However it fails due to error: "failed verification of agent binary: 2 errors occurred".

In particular, we see this error:

2021-06-08T11:04:12.208+0200    ERROR   application/fleet_gateway.go:186        failed to dispatch actions, error: failed to update agent symlink: symlink C:\Program Files\Elastic\Agent\data\elastic-agent-651a2a\elastic-agent.exe C:\Program Files\Elastic\Agent\elastic-agent.exe.prev: Cannot create a file when that file already exists.

Expected result: Successful upgrade

Actual result: Failed upgrade

Workaround: Delete the .prev symlink and restart the elastic agent, then trigger the upgrade again.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)