ci: check out bitcoin/bitcoin PR #35084 instead of master

This repo has introduced API changes to add Windows support to
libmultiprocess (HANDLE-based IPC alongside the existing fd-based IPC).
These changes require corresponding updates to Bitcoin Core, which are
pending in bitcoin/bitcoin#35084. Until that PR merges, the Bitcoin Core
CI jobs fail against master because Bitcoin Core has not yet been updated
to use the new API.

Switch the Bitcoin Core checkout in both jobs to use
refs/pull/35084/merge so CI tests against the compatible version. A
BITCOIN_CORE_REF env var is introduced at the top of the file; once
(and keep the var in place for any future API compatibility cycles).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ryanofsky

.github/workflows/bitcoin-core-ci.yml

@@ -18,6 +18,8 @@ concurrency:
 
 env:
   BITCOIN_REPO: bitcoin/bitcoin
+  # Temporary: use PR #35084 until it merges; revert to refs/heads/master after
+  BITCOIN_CORE_REF: refs/pull/35084/merge
   LLVM_VERSION: 22
   LIBCXX_DIR: /tmp/libcxx-build/
 
@@ -79,6 +81,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ${{ env.BITCOIN_REPO }}
+          ref: ${{ env.BITCOIN_CORE_REF }}
           fetch-depth: 1
 
       - name: Checkout libmultiprocess
@@ -195,6 +198,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ${{ env.BITCOIN_REPO }}
+          ref: ${{ env.BITCOIN_CORE_REF }}
           fetch-depth: 1
 
       - name: Checkout libmultiprocess