.github: build a toolchain for windows/amd64

We will need this for some of our in-progress build/CI improvements.

Updates tailscale/corp#24680

Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

.github/workflows/build.yml

@@ -32,8 +32,11 @@ jobs:
   build_release:
     strategy:
       matrix:
-        GOOS: ["linux", "darwin"]
+        GOOS: ["linux", "darwin", "windows"]
         GOARCH: ["amd64", "arm64"]
+        exclude:
+          - GOOS: windows
+            GOARCH: arm64
     runs-on: ubuntu-20.04
     if: contains(fromJSON('["push", "workflow_dispatch"]'), github.event_name)
     steps: