Update tailscale install script to dynamically get latest stable release

[dseven]

This uses the same method (URL) as the tailscale update CLI, so hopefully it should be reliable.

Up for discission: it'd be more elegant to use jq to parse the JSON, but that'd introduce a new dependency (many Linux systems wouldn't have it installed). Whilst a bit fugly, I think this sed method should be OK. Thoughts or alternative ideas?

[Sam-Horry]

Would love for this to get merged soon!

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❌ adamshiervani
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[adamshiervani]

Needed a tiny fix to work on mac too. Deploying now.