Add a caveat when using CCache in a CI environment

[native-api]

Since the toolchain is git clone'd rather than extracted from a tarball, mtimes of its executables are different each time, invalidating CCache's cache if used in a CI environment.
See e.g. https://travis-ci.community/t/bad-cache-hit-rate-ccache/9400 for an example of this problem.

[lurch]

I'm not sure if this is relevant to our kernel building instructions? Pinging @pelwell for comment
a) I don't think this is specific to compiling the Raspberry Pi kernel
b) surely this is a generic CCache problem, and ccache isn't mentioned anywhere else in our documentation?
c) my comments should be taken with a pinch of salt as I've not used ccache myself 😉

[native-api]

This is relevant to the kernel building instructions because

• they instruct to get the toolchain in an unusual way -- git clone instead of installing a package or unpacking a tarball -- which is the root cause for this problem. It's more relevant to them than CCache documentation because it's them rather than CCache that use an uncommon practice.
• CCache is the standard product to use to save on long compilation times so it's very likely that someone uses it to build the kernel

[pelwell]

It seems like good advice to me.

[JamesH65]

Probably not a huge number of people actually doing this, but seems a sane change nevertheless.

[lurch]

LGTM