When splitting lines for the cross-PR checkout also split on \r\n (#249)

It seems like GitHub sometime in the last year started representing newlines in the PR description using \r\n instead of \n. Respect those as well to split the description into lines and find the linked PR indicator.

Author: ahoppen

.github/workflows/scripts/cross-pr-checkout.swift

@@ -142,7 +142,7 @@ struct CrossRepoPR {
 func getCrossRepoPrs(repository: String, prNumber: String) async throws -> [CrossRepoPR] {
   var result: [CrossRepoPR] = []
   let prInfo = try await getPRInfo(repository: repository, prNumber: prNumber)
-  for line in prInfo.body?.split(separator: "\n") ?? [] {
+  for line in prInfo.body?.split(whereSeparator: \.isNewline) ?? [] {
     guard line.lowercased().starts(with: "linked pr:") else {
       continue
     }