github: Fix API call to request reviews

It used to work, but it stopped (not more than a few days ago, as far
as I can tell) with the following error:

    Type mismatch on variable $pullRequestId and argument
    pullRequestId (String! / ID!)

Perhaps the schema has slightly changed (or type checking is now
tighter)? Anyway, changing the type to `ID!` fixes the issue.

src/cogite/backends/graphql/github/mutation_request_reviews.graphql

@@ -1,5 +1,5 @@
 mutation requestReviews (
-  $pullRequestId: String!,
+  $pullRequestId: ID!,
   $userIds: [ID!],
 ) {
   requestReviews(input: {