workflows: Fix Hubble flow capture in smoke tests

The step "Capture cilium-sysdump" currently fails in smoke tests with:

    gzip: stdin: not in gzip format
    tar: Child returned status 1
    tar: Error is not recoverable: exiting now
    Error: Process completed with exit code 2.

This is happening because we attempt to download the latest hubble
binary at URL github.com/cilium/hubble/releases/download/latest and fail
to decompress the downloaded file. That's because the downloaded file
contains only "Not found".

Indeed, URL cilium/hubble/releases/download/latest doesn't exist, the
correct URL is cilium/hubble/releases/latest/download. Nevermind that
other releases are downloaded at
github.com/cilium/hubble/releases/download/$TAG ¯\_(ツ)_/¯

Fixes: 0cbb855 (".github: Capture hubble flows when smoke test fails")
Signed-off-by: Paul Chaignon <paul@cilium.io>

.github/workflows/tests-smoke-ipv6.yaml

@@ -133,7 +133,7 @@ jobs:
         # file (EOF) on stdin and displaying no flows.
         shell: 'script -q -e -c "bash --noprofile --norc -eo pipefail {0}"'
         run: |
-          version="$(curl -sL0 'https://api.github.com/repos/cilium/hubble/releases/latest' | jq -r .tag_name)"
+          version=$(curl -s https://raw.githubusercontent.com/cilium/hubble/master/stable.txt)
           curl -sLO "https://github.com/cilium/hubble/releases/download/${version}/hubble-linux-amd64.tar.gz"
           tar zxf hubble-linux-amd64.tar.gz
           chmod +x ./hubble

.github/workflows/tests-smoke.yaml

@@ -182,7 +182,8 @@ jobs:
         # file (EOF) on stdin and displaying no flows.
         shell: 'script -q -e -c "bash --noprofile --norc -eo pipefail {0}"'
         run: |
-          curl -sLO https://github.com/cilium/hubble/releases/download/latest/hubble-linux-amd64.tar.gz
+          version=$(curl -s https://raw.githubusercontent.com/cilium/hubble/master/stable.txt)
+          curl -sLO "https://github.com/cilium/hubble/releases/download/${version}/hubble-linux-amd64.tar.gz"
           tar zxf hubble-linux-amd64.tar.gz
           chmod +x ./hubble
           touch hubble-flows.json