fix: pin Boost to 1.85 on macOS to fix foxy compilation

The vendored foxy library is incompatible with Boost 1.90 (installed
by default on macos-15 via Homebrew). Pin to boost@1.85 until foxy
is updated to work with the newer Boost version.

Co-Authored-By: rlamb@launchdarkly.com <kingdewman@gmail.com>

Author: devin-ai-integration[bot]

.github/actions/install-boost/action.yml

@@ -43,8 +43,8 @@ runs:
       if: runner.os == 'macOS'
       shell: bash
       run: |
-        brew install boost
-        echo "BOOST_ROOT=$(brew --prefix boost)" >> $GITHUB_OUTPUT
+        brew install boost@1.85
+        echo "BOOST_ROOT=$(brew --prefix boost@1.85)" >> $GITHUB_OUTPUT
 
     - name: Determine root
       id: determine-root