.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.tailscale-HEAD

@@ -37,7 +37,7 @@ RUN GOARCH=$TARGETARCH go install -tags="${BUILD_TAGS}" -ldflags="\
       -v ./cmd/tailscale ./cmd/tailscaled ./cmd/containerboot
 
 FROM alpine:3.22
-RUN apk add --no-cache ca-certificates iptables iproute2 ip6tables curl
+RUN apk add --no-cache ca-certificates iptables iproute2 ip6tables curl traceroute
 
 COPY --from=build-env /go/bin/* /usr/local/bin/
 # For compat with the previous run.sh, although ideally you should be