Install boost into /usr/local in CI

Install boost into /usr/local on macOS in CI, as /usr/{lib,include,bin,etc.} are read-only

.github/composite-actions/download-libraries/action.yml

@@ -123,13 +123,14 @@ runs:
       if: inputs.install-boost-clang != 'false'
     - name: Install Boost built with Homebrew Clang
       # b2 doesn't recognize custom compilers, so some trick is needed:
+      # Also, /usr is read-only, so install into /usr/local
       run: |
         cd lib/boost
         export PATH=$(brew --prefix llvm@17)/bin:$PATH
-        ./bootstrap.sh --prefix=/usr --with-toolset=clang
+        ./bootstrap.sh --prefix=/usr/local --with-toolset=clang
         ./b2 clean
         ./b2 toolset=clang cxxflags="-std=c++11 -stdlib=libc++" linkflags="-stdlib=libc++"
-        sudo ./b2 install --prefix=/usr
+        sudo ./b2 install --prefix=/usr/local
       shell: bash
       if: inputs.install-boost-brew-clang != 'false'
     - name: Download frozen

.github/workflows/core-tests.yml

@@ -118,6 +118,7 @@ jobs:
         run: |
           export CC=$(brew --prefix llvm@17)/bin/clang
           export CXX=$(brew --prefix llvm@17)/bin/clang++
+          export BOOST_ROOT=/usr/local
           if [[ "${{matrix.cfg.BUILD_TYPE}}" == "Debug" ]]; then
             ./build.sh --debug --sanitizer=${{ matrix.cfg.SANITIZER }}
           else