chore: document gcovr pip install requirement

Co-authored-by: pavel-kirienko <3298404+pavel-kirienko@users.noreply.github.com>

Author: Copilot

toolshed/Dockerfile

@@ -161,6 +161,7 @@ RUN echo "export PATH=$PIPX_HOME:$PIPX_BIN_DIR:$PATH" >> ~/.bashrc
 RUN pipx install cowsay
 RUN pipx install tox
 RUN pipx install nox
+# gcovr is required via pip with --break-system-packages for toolshed usage.
 RUN pip install --break-system-packages gcovr
 RUN pipx install yakut
 RUN cowsay -t moo