ci(eks-ami): build on self-hosted ci-single runner (#326)

* ci(eks-ami): build on self-hosted ci-single-1 runner

GitHub-hosted ubuntu-24.04 cannot run the libguestfs appliance network:
passt fails (PID file denied under its AppArmor confine / privilege drop)
and the SLIRP fallback has no outbound, so virt-customize --network dies.
The self-hosted ci-single-1 runner has /dev/kvm and real networking, so
the existing libguestfs build works unmodified.

* fix(build-system-image): add --no-cache, force fresh build on publish

The <10min cached-raw skip avoids duplicate work for concurrent e2e
jobs, but on the persistent publish runner it could republish a stale
image from a prior or concurrent run. Add --no-cache to bypass the skip
and have publish-system-image.sh pass it on every build.

* ci(eks-ami): target ci-single label (no ci-single-1 label exists)

The self-hosted runners are named ci-single/-2/-3 but all share the
single label 'ci-single'; runs-on matches labels, not names, so
ci-single-1 never schedules.

* ci(eks-ami): disable Go cache on self-hosted runner

setup-go's cache-save (Post Setup Go) re-uploaded the Go cache to the
Actions backend on every run, taking ~2.5min vs the 36s build. The
self-hosted runner already keeps the cache on local disk, so cache:false.

.github/workflows/publish-eks-ami.yml

@@ -26,7 +26,7 @@ concurrency:
 jobs:
   publish:
     name: Build + publish eks-node AMI
-    runs-on: ubuntu-24.04
+    runs-on: [ self-hosted, ci-single ]
 
     steps:
       - name: Checkout Spinifex
@@ -51,8 +51,12 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v6
         with:
-          cache-dependency-path: "spinifex/go.sum"
           go-version-file: "spinifex/go.mod"
+          # Self-hosted runner: the Go module + build cache already persists on
+          # local disk between runs. setup-go's cache-save ("Post Setup Go")
+          # otherwise re-uploads hundreds of MB to the Actions cache backend,
+          # taking longer than the build itself.
+          cache: false
 
       - name: Setup Go Workspace
         run: go work init ./spinifex ./viperblock ./predastore