message: Fix CI builds for macos-gcc & macos-clang
  As per the reported issue here,
  /usr/local on macos Sierra is a protected location.
  Use location relative to the workspace to guarantee we don't
  run into permission related issues again.
diff: @@ -355,7 +355,7 @@ jobs:
  echo 'CC=gcc-9' >> $GITHUB_ENV
  echo 'CXX=g++-9' >> $GITHUB_ENV
  echo "PATH=$(brew --prefix)/opt/ccache/libexec:$PATH" >> $GITHUB_ENV
  - echo 'PREFIX=/usr/local' >> $GITHUB_ENV
  + echo 'PREFIX=${GITHUB_WORKSPACE}/install' >> $GITHUB_ENV
  - name: Show shell configuration
  run: |
  @@ -372,7 +372,7 @@ jobs:
  - name: Build
  run: |
  make release
  - sudo make install
  + make install
  - name: Unit tests
  run: |
  @@ -409,7 +409,7 @@ jobs:
  - name: Configure shell
  run: |
  - echo 'PREFIX=/usr/local' >> $GITHUB_ENV
  + echo 'PREFIX=${GITHUB_WORKSPACE}/install' >> $GITHUB_ENV
  echo "PATH=$(brew --prefix)/opt/ccache/libexec:$PATH" >> $GITHUB_ENV
  - name: Show shell configuration
  @@ -425,7 +425,7 @@ jobs:
  - name: Build
  run: |
  make release
  - sudo make install
  + make install
  - name: Unit tests
  run: |
  