Add `balena update` command for updating the CLI

[myarmolinsky]

Add balena update command for updating the CLI

Resolves: #1669 Change-type: minor

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Please check the CONTRIBUTING.md file for relevant information and some guidance. Keep in mind that the CLI is a cross-platform application that runs on Windows, macOS and Linux. Tests will be automatically run by balena CI on all three operating systems, but this will only help if you have added test code that exercises the modified or added feature code.

Note that each commit message (currently only the first line) will be automatically copied to the CHANGELOG.md file, so try writing it in a way that describes the feature or fix for CLI users.

If there isn't a linked issue or if the linked issue doesn't quite match the PR, please add a PR description to explain its purpose or the features that it implements. Adding PR comments to blocks of code that aren't self explanatory usually helps with the review process.

If the PR introduces security considerations or affects the development, build or release process, please be sure to highlight this in the PR description.

Thank you very much for your contribution!

[thgreasi]

If you confirm that

• this works for both npm & standalon installations
• that the product team doesn't want to keep the update command available from something else in the future

then LGTM (but there isn't anything to look tbh, given that there are no code changes).