Update windows install script to detect arm64 systems

[yob]

In #1767 we added windows/arm64 to the official build matrix for the agent. However, the recommended powershell install script in our docs looks like this:

PS> $env:buildkiteAgentToken = "<your_token>"
PS> Set-ExecutionPolicy Bypass -Scope Process -Force
iex ((New-Object System.Net.WebClient).DownloadString('https://raw.githubusercontent.com/buildkite/agent/main/install.ps1'))

.. and it always decides what binary to download by fetching this URL with a hard coded amd64 arch:

$ curl "https://buildkite.com/agent/releases/latest?platform=windows&arch=amd64"

version=3.39.0
filename=buildkite-agent-windows-amd64-3.39.0.zip
url=https://github.com/buildkite/agent/releases/download/v3.39.0/buildkite-agent-windows-amd64-3.39.0.zip

This PR updates the install.ps1 script to detect the architecture of the local machine and send it in the request to buildkite.com. Currently that returns no results:

$ curl
"https://buildkite.com/agent/releases/latest?platform=windows&arch=arm64"

No release found for these parameters

However, once #1767 is shipped in an official release (presumably 3.40.0) then it should Just Work (like this):

$ curl "https://buildkite.com/agent/releases/latest?platform=windows&arch=arm64"

version=3.40.0
filename=buildkite-agent-windows-arm64-3.40.0.zip
url=https://github.com/buildkite/agent/releases/download/v3.40.0/buildkite-agent-windows-arm64-3.40.0.zip

... and powershell will automatically install a binary with the correct architecture.