tools: do not mask possible error from command in start-ci.sh

Revise start-ci.sh to conform with shellcheck 0.8.0 default checks.

Refs: https://www.shellcheck.net/wiki/SC2312

Author: Trott

tools/actions/start-ci.sh

@@ -17,7 +17,8 @@ for pr in "$@"; do
     # Do we need to reset?
     gh pr edit "$pr" --add-label "$REQUEST_CI_FAILED_LABEL"
 
-    jq -n --arg content "<details><summary>Couldn't start CI</summary><pre>$(cat output)</pre></details>" > output.json
+    details="$(cat output)"
+    jq -n --arg content "<details><summary>Couldn't start CI</summary><pre>$details</pre></details>" > output.json
 
     gh pr comment "$pr" --body-file output.json