ci: cache dependencies to speed up build time

[henryborchers]

No description provided.

[JeromeMartinez]

Speed up confirmed with x2 to x5 speed up depending on the OS.
Thanks.

[henryborchers]

I honestly was hoping to figure out why core.cpp takes so long to compile into an object file in certain cases.

It's frustrating because in some environments, it takes seconds and in others it takes minutes. I ran gcc with the -ftime-report on one of my very slow environments and saw that it was using more than 700 mb for "callgraph functions expansion" phase.

[JeromeMartinez]

in some environments, it takes seconds and in others it takes minutes.

We are not motivated to manage that on our side because it does not impact us much but if you find a patch for fixing that we would take it.

[henryborchers]

We are not motivated to manage that on our side because it does not impact us much but if you find a patch for fixing that we would take it.

Completely understand.

This PR was motivated by me get faster results to see if I could figure it out. I just ran out of time to work on it and I figured that having a fast CI results would at least be a benefit to you in the meantime.