Document and make recommendations for additional sources of package variance

This issue is a bit of a catchall but I have a specific example.

First of all the [guidelines](https://docs.conan.io/2/knowledge/guidelines.html) say "Sources used in recipes must be immutable". I'm not sure how that squares with anything that is versioned cci.latest (e.g. msys2).

Speaking of msys2:
* Only one copy is shared in the cache. Therefore, changes made to it's configuration by one package can affect the building of another package. This I suppose also applies to any subsystem you're building on.
* If packages need to be installed, there should be a recommendation to install a specific version of the package (this applies to any subsystem I suppose).
* Then there is the dependency on the package manager itself...a pretty hard problem to solve, but I think it would be good if people are at least explicitly aware.

Some of these issues could be fixed with e.g. a separate container instance to build each package binary where the base configuration is known and consistent (again specifying versions for all subsystem packages).

Anyhow, the general idea is to at a minimum start identifying all these issues, and even better recommending best practices to mitigate them.