Windows install script

[lox]

This adds a powershell install script so that we can have users do windows installs with:

$env:buildkiteAgentToken = "my_token"
$env:buildkiteAgentBeta = 'true'
Set-ExecutionPolicy Bypass -Scope Process -Force
iex ((New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/buildkite/agent/master/install.ps1'))

[keithpitt]

Nice! Would we still bundle the service installer with the downloadable release?

[lox]

Nah I don't think so, it's 3 lines and it's using a custom service manager. We can document that bit, this just makes the install really easy.

[lox]

Needs some more niceties from https://chocolatey.org/install.ps1.

[lox]

But it's a start!