Remove ccache from CI

[luca-della-vedova]

Do you know why CI has been flaky for ages?

Because specifying --cmake-args after the mixin overwrote the release mixin and we were building in debug mode.

There.

[aaronchongth]

Thanks for the fix! LGTM

[Yadunund]

Why remove ccahce? Wouldn't this lead to slower builds?

[luca-della-vedova]

We can reintroduce it by using the mixin but I don't think it actually does anything so I'm a bit puzzled at why it was there in the first place.

Ccache is supposed to speed up recompilation but when we are in CI we start from a blank image so we need to compile everything from scratch anyway and ccache doesn't do anything.

To prove you can see the various attempts of this PR's CI that take around 23 mins, that is consistent with other PRs CI.

I realized I forgot to remove the ccache CI step in the cleanup but that seems to take 1 second anyway

[aaronchongth]

Yeah looking at the build-times, I think we weren't using cccahe together with the action/cache properly.

From some quick searches, it looks like the steps required to restore the cache is a little more convoluted than what we were doing. I'd be happy to take a crack at it when I'm done with the rmw migration, but hopefully by then the compilation times would have sped up enough to not be an issue already