Fix yarn es snapshot cache hit (#161183)

The `yarn es` command provides a useful `--use-cached` to avoid
redownloading the cache, but this is always skipped because no one set
the `exists` field is required by the command to check if the cache is
available.

This commit just adds that `exists` property where is needed (when reading
the `.meta` file) and leaves the rest of the code unaltered.
I could have implemented the same behavior by returning `null` from the
`readMeta` and doing a null check where we checked for the `exists`
property. Please feel free which approach you feel better.

packages/kbn-es/src/utils/cache.ts

@@ -18,13 +18,16 @@ export const cache = {
 
       return {
         ...JSON.parse(meta),
+        exists: true,
       };
     } catch (e) {
       if (e.code !== 'ENOENT') {
         throw e;
       }
 
-      return {};
+      return {
+        exists: false,
+      };
     }
   },