Fix regression

While trying to be type correct, coercing the value to string has the unintended side effect of allowing regexes (or any other type) as $regex arg. So instead now throwing an explciit error for that case.

Also reverted the type where it returns an Array. Apparently there are cases where it is not an array.

packages/gatsby/src/redux/run-fast-filters.ts

@@ -85,7 +85,7 @@ function createFilterCacheKey(
 
 function prepareQueryArgs(
   filterFields: Array<IInputQuery> | IInputQuery = {}
-): Array<IPreparedQueryArg> {
+): IPreparedQueryArg {
   const filters = {}
   Object.keys(filterFields).forEach(key => {
     const value = filterFields[key]
@@ -96,7 +96,12 @@ function prepareQueryArgs(
     } else {
       switch (key) {
         case `regex`:
-          filters[`$regex`] = prepareRegex(String(value))
+          if (typeof value !== `string`) {
+            throw new Error(
+              `The $regex comparator is expecting the regex as a string, not an actual regex or anything else`
+            )
+          }
+          filters[`$regex`] = prepareRegex(value)
           break
         case `glob`:
           filters[`$regex`] = makeRe(value)