feat: add linux/arm64 build (#72)

[antoinetoussaint-byte]

Closes#72.
Refs codefly-dev/core#346.

Summary

• Adds a linux/arm64 goreleaser build so the agent publishes service-python_<version>_linux_arm64.tar.gz — without it, agent resolution fails at runtime on any arm64 Linux target (core's downloader already asks for that asset by host arch).
• cgo-only change concern is covered: the release already runs under goreleaser-cross, which ships aarch64-linux-gnu-gcc; this stanza just points CC/CXX at it. No release-workflow change needed.

Test plan

• goreleaser check passes
• Merge, then cut a new release (tag) so CI cross-builds and publishes the asset
• Confirm the asset exists: gh release view v<version> --repo codefly-dev/service-python --json assets --jq '[.assets[].name | select(contains("linux_arm64"))]'

Note

This PR only adds the build target; a maintainer must cut a release for the linux_arm64 asset to actually publish. I can't push a release tag.