Compile the Windows archive proxy for the target GOARCH instead of the host GOARCH

[cmacknz]

• Closes archive-proxy is compiled for the host architecture and not the target architecture #10965

When support for windows/arm builds was added in #9751 the Go archive proxy was switched from a fixed amd64 GOARCH to the host GOARCH. This works in CI which doesn't cross-compile for Windows but broke cross-compiling from darwin/arm64 to windows/amd64 for anyone on a Mac. This PR fixes that.

To test:

❯ EXTERNAL=true SNAPSHOT=true PACKAGES=zip PLATFORMS=windows/amd64,windows/arm64 mage package

# ...

❯ file build/distributions/elastic-agent-9.3.0-SNAPSHOT-windows-x86_64/elastic-agent.exe
build/distributions/elastic-agent-9.3.0-SNAPSHOT-windows-x86_64/elastic-agent.exe: PE32+ executable (console) x86-64, for MS Windows

❯ file build/distributions/elastic-agent-9.3.0-SNAPSHOT-windows-arm64/elastic-agent.exe
build/distributions/elastic-agent-9.3.0-SNAPSHOT-windows-arm64/elastic-agent.exe: PE32+ executable (console) Aarch64, for MS Windows

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[ycombinator]

Just a comment update, otherwise LGTM

[elasticmachine]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: dcc67d8

Failed CI Steps

• Win2025:sudo:fleet
• x86_64:sudo: default

cc @cmacknz