I wanted to build elastic-agent and be able to build everything from HEAD.

While I was able to have elastic-agent and beats working, I noticed that the shipper component was missing.
After talking with @leehinman he verified that using the dropin works, but I need to build from source.

For confirmed bugs, please report:

• Version: 8.8.0
• Operating System: 18.04.6 LTS (Bionic Beaver)
• Steps to Reproduce:

1. Checkout elastic-agent
2. Checkout beats
3. use `EXTERNAL=false SNAPSHOT=true PACKAGE=tar.gz DEV=true PLATFORMS=linux/amd64 mage package
4. examine the resulted artifact.