Fix spotless ratcheting on release branches (#1401)

buildSrc/src/main/kotlin/pklAllProjects.gradle.kts

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -142,9 +142,17 @@ private fun KotlinGradleExtension.configureFormatter() {
 }
 
 val originalRemoteName = System.getenv("PKL_ORIGINAL_REMOTE_NAME") ?: "origin"
+// if we're running against a release branch (or a PR targeted at one), use that branch for
+// ratcheting
+// these env vars are set by GitHub actions:
+// https://docs.github.com/en/actions/reference/workflows-and-actions/variables#default-environment-variables
+val ratchetBranchName =
+  (System.getenv("GITHUB_BASE_REF") ?: System.getenv("GITHUB_REF_NAME"))?.let {
+    if (it.startsWith("release/")) it else null
+  } ?: "main"
 
 spotless {
-  ratchetFrom = "$originalRemoteName/main"
+  ratchetFrom = "$originalRemoteName/$ratchetBranchName"
 
   // When building root project, format buildSrc files too.
   // We need this because buildSrc is not a subproject of the root project, so a top-level