.github/workflows: install promtool from binary release

`go get` no longer works to install binaries with Go 1.18 and `go install`
unfortunately doesn't work due to
prometheus/prometheus#8852 and related issues.

Thus install promtool from the Prometheos binary release. While at it
bump the version to v2.34.0.

Signed-off-by: Tobias Klauser <tobias@cilium.io>

.github/workflows/tests-smoke.yaml

@@ -20,6 +20,7 @@ env:
   CONFORMANCE_TEMPLATE: examples/kubernetes/connectivity-check/connectivity-check.yaml
   TIMEOUT: 2m
   LOG_TIME: 30m
+  PROM_VERSION: 2.34.0
 
 jobs:
   check_changes:
@@ -187,8 +188,11 @@ jobs:
           cilium_pod=$(kubectl -n kube-system get po -o name --field-selector=status.phase==Running -l 'k8s-app=cilium' -o jsonpath='{.items[0].metadata.name}' )
           kubectl -n kube-system exec $cilium_pod -- sh -c "apt update && apt install curl -y"
           kubectl -n kube-system exec $cilium_pod -- curl http://localhost:9090/metrics > metrics.prom
-          go install github.com/prometheus/prometheus/cmd/promtool@v2.31.1
-          cat metrics.prom | $HOME/go/bin/promtool check metrics
+          curl -sSLO https://github.com/prometheus/prometheus/releases/download/v${PROM_VERSION}/prometheus-${PROM_VERSION}.linux-amd64.tar.gz
+          tar xzvf prometheus-${PROM_VERSION}.linux-amd64.tar.gz prometheus-${PROM_VERSION}.linux-amd64/promtool
+          rm -f prometheus-${PROM_VERSION}.linux-amd64.tar.gz
+          sudo mv prometheus-${PROM_VERSION}.linux-amd64/promtool /usr/bin
+          cat metrics.prom | promtool check metrics
 
       - name: Capture cilium-sysdump
         if: ${{ failure() }}