chore(gatsby-source-contentful): cache the cache key (#28444)

Author: pvdz

packages/gatsby-transformer-remark/src/extend-node-type.js

@@ -364,15 +364,16 @@ module.exports = function remarkExtendNodeType(
     }
 
     async function getHTMLAst(markdownNode) {
-      const cachedAst = await cache.get(htmlAstCacheKey(markdownNode))
+      const key = htmlAstCacheKey(markdownNode)
+      const cachedAst = await cache.get(key)
       if (cachedAst) {
         return cachedAst
       } else {
         const ast = await getAST(markdownNode, cache)
         const htmlAst = markdownASTToHTMLAst(ast)
 
         // Save new HTML AST to cache and return
-        await cache.set(htmlAstCacheKey(markdownNode), htmlAst)
+        await cache.set(key, htmlAst)
         return htmlAst
       }
     }