Support running the E2E against the local artifacts generated by the Beats repository

[mdelapenya]

It would be a nice-to-have to run the E2E against the Beats locally generated artifacts, improving the developer experience.

To generate the artifacts:

export THE_BEAT="elastic-agent"
cd x-pack/$THE_BEAT; SNAPSHOT=true PLATFORMS=darwin   DEV=true mage package

for docker images set platform:

PLATFORMS="+all linux/amd64"

An initial idea could be passing an environment variable with the location of the local Beats repo, so that it's possible to get the artifacts from that path. Something like BEATS_LOCAL_PATH=/tmp/foo

@elastic/observablt-robots @michalpristas @EricDavisX @ph, thoughts?

[mdelapenya]

In the conversation we mentioned the idea of adding a command in Beats' build system, so that it fetches current version of the E2E tests (per maintenance branch) and automatically triggers them.