acloud: Mkcert install failed

Mkcert install process fail caused by mkcert directory existed.
Add a check to solve the problem

Bug: 195474146
Test: acloud-dev create --autoconnect webrtc --local-image
--local-instance

Change-Id: If84b43b31e6037d512a4495af2ecae2ceccb0cd5
1 file changed