build agent binaries for windows/arm64
build agent binaries for windows/arm64

This adds arm64 windows binaries to our official build matrix.
Originally requested in #1282, but at the time we were nervous about
enabling because:

* none of us had access to windows/arm64 systems
* it required a golang 1.17+, and we were compiling the agent with 1.16

These days we're compiling the agent with golang 1.18 so windows/arm64
is a target that Just Works. An arm64 windows environment is also super
easy to boot using parallels 18 on an Apple Silicon machine, and we have
plenty of those.

I tested this by manually cross compiling compiling a windows/arm64 binary
on my linux laptop and then using it inside an arm parallels VM.

Compilation on Linux:

    GOOS=windows GOARCH=arm64 go build -o buildkite-agent-win-arm64 .

Fixes #1282

buildkite/agent,buildkite/agent,buildkite/agent