Properly set Go version in CI

[cmacknz]

Don't unconditionally build with Go version 1.18.2 by reading the Go version to use from the .go-version file. Also make sure the notice file is regenerated so mage check will detect when someone added a dependency but didn't update the license file.

Port changes from elastic/elastic-agent-shipper#45

[elasticmachine]

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• Start Time: 2022-06-06T21:24:57.280+0000

• Duration: 9 min 33 sec

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Test	Results
Failed	0
Passed	1067
Skipped	6
Total	1073

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