Use main in RAPIDS_BRANCH (#1439)

Supports rollout of new branching strategy. https://docs.rapids.ai/notices/rsn0047/

xref: rapidsai/build-planning#224

Authors:
  - Bradley Dice (https://github.com/bdice)
  - Nate Rock (https://github.com/rockhowse)

Approvers:
  - Robert Maynard (https://github.com/robertmaynard)
  - Nate Rock (https://github.com/rockhowse)

URL: #1439

Author: bdice

.github/workflows/build.yaml

@@ -3,7 +3,7 @@ name: build
 on:
   push:
     branches:
-      - "branch-*"
+      - "main"
     tags:
       - v[0-9][0-9].[0-9][0-9].[0-9][0-9]
   workflow_dispatch:

RAPIDS_BRANCH

@@ -1 +1 @@
-branch-25.12
+main