.github/workflows: prebuilt integration test artifacts (#2954)

This PR restructures the integration tests and prebuilds all common assets used in all tests:

Headscale and Tailscale HEAD image
hi binary that is used to run tests
go cache is warmed up for compilation of the test
This essentially means we spend 6-10 minutes building assets before any tests starts, when that is done, all tests can just sprint through.

It looks like we are saving 3-9 minutes per test, and since we are limited to running max 20 concurrent tests across the repo, that means we had a lot of double work.

There is currently 113 checks, so we have to do five runs of 20, and the saving should be quite noticeable! I think the "worst case" saving would be 20+min and "best case" probably towards an hour.

Dockerfile.integration-ci

@@ -0,0 +1,17 @@
+# Minimal CI image - expects pre-built headscale binary in build context
+# For local development with delve debugging, use Dockerfile.integration instead
+
+FROM debian:trixie-slim
+
+RUN apt-get --update install --no-install-recommends --yes \
+    less jq sqlite3 dnsutils ca-certificates procps bash findutils curl traceroute \
+  && apt-get dist-clean
+
+RUN mkdir -p /var/run/headscale
+
+# Copy pre-built headscale binary from build context
+COPY headscale /usr/local/bin/headscale
+
+ENTRYPOINT []
+EXPOSE 8080/tcp
+CMD ["/usr/local/bin/headscale"]