Switch to GitHub-hosted arm64 runners

[dennisameling]

GitHub-hosted arm64 runners are now in General Availability for teams/enterprise. Support for open source projects is following later this year.

Given that git-for-windows is a GitHub org, you may already be able to leverage the paid hosted runners, in favor of the current setup where we have to configure VMs in Azure ourselves. Or you could wait until the GitHub-hosted arm64 runners become free for open source projects.

Some related notes:

• Here's 30 successful workflow runs with this setup.
• The hosted image provided by ARM doesn't contain any tools like Git or MSVS yet. It looks like a new image is coming soon that will have a bunch of tools preinstalled. For now, I just modified the post-deployment-script.ps1 a bit to install GfW and pwsh manually.
• Setting up the runners in my dennisameling-org was pretty straightforward. They're priced at $0.02 per minute, pay-as-you-go. The pricing page says there's a 2-core option, but the smallest I could select was 4-core. I'm curious how the pricing compares to the self-hosted Azure VMs that we're using now.