Make the shared ccache optional

[cottsay]

It isn't obvious or expected that running buildfarm tasks locally (outside of Jenkins) could modify the local ccache of the user invoking the command. This change disables ccache sharing by default. It can be re-enabled using the newly added shared_ccache build file option.

[nuclearsandwich]

Thanks for making sure the new option is documented. Looks good to me.

[mathias-luedtke]

This PR disables the ccache sharing by default..
Where did you set the option to true for the buildfarm?

And am I right that there is no way to enable it for pre-release tests?

[cottsay]

This is disabled on build.ros.org and build.ros2.org.

The approach implemented in ros_buildfarm shares the cache among ALL packages and ALL jobs. We've concluded that the benefits provided by ccache do not outweigh the risks of a misconfigured or malicious package poisoning that global cache.

Additional development is necessary to either secure the cache by enforcing that only authorized (signed) compilers are contributing to it, or making the cache local to only the package or job. Right now, we don't have any plans to implement either of those features. Pull requests are welcome, as always.

[mathias-luedtke]

Thanks for the clarification!

I will try to add a CLI option for the generation scripts..