build: pin build-docs workflow to Node.js 14

The build-docs workflow runs tests which means it is sensitive to
globals and can fail if the `node` running the test has a different
set of globals to that expected by `test/common`.

PR-URL: #40939
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

.github/workflows/misc.yml

@@ -11,7 +11,7 @@ on:
       - v[0-9]+.x
 
 env:
-  NODE_VERSION: lts/*
+  NODE_VERSION: 14
 
 jobs:
   build-docs: