ci: Check out bitcoin/bitcoin PR #35454 instead of master

bitcoin-core#287 is removing the mpcalculator,
mpprinter, and mpexample targets with causes an error in the Bitcoin
Core CI jobs without bitcoin/bitcoin#35454.

Switch the Bitcoin Core checkout in both jobs to use
refs/pull/35454/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

@@ -23,6 +23,8 @@ on:
 
 env:
   BITCOIN_REPO: bitcoin/bitcoin
+  # Temporary: use PR #35454 until it merges; revert to refs/heads/master after
+  BITCOIN_CORE_REF: refs/pull/35454/merge
   LLVM_VERSION: 22
   LIBCXX_DIR: /tmp/libcxx-build/
 
@@ -84,6 +86,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ${{ env.BITCOIN_REPO }}
+          ref: ${{ env.BITCOIN_CORE_REF }}
           fetch-depth: 1
 
       - name: Checkout libmultiprocess
@@ -209,6 +212,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: ${{ env.BITCOIN_REPO }}
+          ref: ${{ env.BITCOIN_CORE_REF }}
           fetch-depth: 1
 
       - name: Checkout libmultiprocess