ci/request-reviews: Don't request reviews from users not part of the org

Fixes this problem for maintainer-based reviews when the maintainer
didn't yet accept or missed the automated invite:

    gh: Reviews may only be requested from collaborators. One or more of the users or teams you specified is not a collaborator of the NixOS/nixpkgs repository. (HTTP 422)

ci/request-reviews/process-reviewers.sh

@@ -19,6 +19,8 @@ baseRepo=$1
 prNumber=$2
 prAuthor=$3
 
+org=${baseRepo%%/*}
+
 tmp=$(mktemp -d)
 trap 'rm -rf "$tmp"' exit
 
@@ -47,6 +49,16 @@ while read -r user; do
     fi
 done < "$tmp/already-reviewed-by"
 
+for user in "${!users[@]}"; do
+    if ! gh api \
+        -H "Accept: application/vnd.github+json" \
+        -H "X-GitHub-Api-Version: 2022-11-28" \
+        "/orgs/$org/members/$user" >&2; then
+        log "User $user is not part of the $org org, probably missed the automated invite (see https://github.com/NixOS/nixpkgs/issues/234293), ignoring.."
+        unset 'users[$user]'
+    fi
+done
+
 # Turn it into a JSON for the GitHub API call to request PR reviewers
 jq -n \
     --arg users "${!users[*]}" \