fix: sync browser version in docker and examples

[vigneshshanmugam]

• fix Run Playwright post-install script to fix chrome binary sometimes unavailable #142
• Forcefully reinstalls our @elastic/synthetics package by ignoring the cache incase any versions have changed. We have not used the docker build --no-cache as it will also clear out other dependency caches
• By having the latest version on both official docker image and examples pointing to the latest published version. We synchronise the playwright versions inside the docker image and do not have a broken state.
• We can also remove the intermin solution that is put inside the heartbeat code https://github.com/elastic/beats/blob/heartbeat-synthetics/x-pack/heartbeat/monitors/browser/synthexec/synthexec.go#L57 as the problem is not related to playwright being available instead a broken state.

Steps to test this flow.

// remove the previous docker image if you want to test in a clean environment
docker rmi heartbeat-synthetics-local
npm run docker 
cd examples/docker
./run-build-local.sh -E output.console={}

[apmmachine]

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Build stats

• Build Cause: [Pull request #147 opened]

• Start Time: 2020-11-24T16:03:08.849+0000

• Duration: 13 min 15 sec

Test stats 🧪

Test	Results
Failed	0
Passed	45
Skipped	0
Total	45

[paulb-elastic]

I've run this locally and it works, I'm getting the JSON output from the synthetic monitors that are running. I had to authorise with the docker registry before this would work.

[vigneshshanmugam]

Will revisit later with better alternatives if we decide to do docker cache cleanup at some point for building new images.