Use boost 1.86 - 1.87 removes deprecated API

.github/actions/setup-build-agent/setup_gh_actions.sh

@@ -60,7 +60,17 @@ else
     brew install ccache
 
     if [ "$TARGET" = "shared" ]; then
-        brew install boost
+        # Boost 1.87 removes certain deprecated APIs that we used to depend on.
+        # A patch is provided in Botan 3.7.0, in order to allow building 3.6.1,
+        # we explicitly install the previous version of boost.
+        #
+        # See also: https://github.com/randombit/botan/pull/4477
+        #           https://github.com/randombit/botan/pull/4484
+        #
+        # TODO: remove this as soon as we are done with Botan 3.6.1.
+        brew search boost # for debugging
+        brew install [email protected]
+        brew link --force --overwrite [email protected]
 
         # On Apple Silicon we need to specify the include directory
         # so that the build can find the boost headers.