ci: add the trailing newline the redirect canary needs

The canary failed with exit 1 and printed nothing at all, which looked
like a redirect finding and was not. `curl -w` emits no trailing newline,
and `read` returns non-zero at EOF even when it has assigned every
variable, so `set -e` killed the step before its first echo.

Reproduced directly:

  read -r a b < <(printf 'X Y')    -> exit 1, no output
  read -r a b < <(printf 'X Y\n')  -> exit 0, "a=X b=Y"

Adds `\n` to the -w format and a comment recording why it is load-bearing,
since the failure mode is silent and easy to reintroduce.

Signed-off-by: Mark Chmarny <mark@chmarny.com>

Author: mchmarny

.github/workflows/dgxc-goproxy-probe.yaml

@@ -164,10 +164,14 @@ jobs:
           auth="$("${GOAUTH}" | grep -i '^Authorization:')"
           [ -n "${auth}" ] || { echo "::error::GOAUTH helper produced no Authorization header"; exit 1; }
 
+          # The trailing newline in -w is load-bearing. `read` returns non-zero
+          # at EOF even when it assigned every variable, so without it `set -e`
+          # kills this step with no output at all — which is exactly how it
+          # failed the first time.
           read -r redirects effective < <(
             printf 'header = "%s"\nurl = "%s"\n' "${auth}" "${url}" \
               | curl --config - -sSL --max-time 60 -o /dev/null \
-                     -w '%{num_redirects} %{url_effective}'
+                     -w '%{num_redirects} %{url_effective}\n'
           )
           echo "redirects: ${redirects}"
           echo "final URL host: $(printf '%s' "${effective}" | sed -E 's#^https?://([^/]+)/.*#\1#')"