[Uptime] Copy synthetics suites to tmpdir before running

[andrewvc]

The rationale here is that doing so resolves any file permissions issues that may be present due to the suite directory being shared to the container as read only OR due to incompatible UIDs between the docker container and the host.

Fixes elastic/synthetics#156

As a note, no tests are added here due to the complexity of testing this small amount of I/O functionality, however, any issues should be caught by our E2E tests in https://github.com/elastic/synthetics/tree/master/__tests__/e2e . I've opened an issue to improve this situation here: #23346

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

You can test this manually by running mage build && ./heartbeat -e --path.config sample-synthetics-config/ and ensuring that there's still output.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[elasticmachine]

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Build stats

• Build Cause: Pull request #23347 updated

• Start Time: 2021-01-04T20:36:17.970+0000

• Duration: 22 min 17 sec

Test stats 🧪

Test	Results
Failed	0
Passed	953
Skipped	13
Total	966

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Test stats 🧪

Test	Results
Failed	0
Passed	953
Skipped	13
Total	966

[andrewvc]

Closing in favor of #23467