Use a new prefix when using the MSVC toolchain

commit c5a99b14:

cerbero: Load command-line variants after user config
So that `-v x11` will override `nox11` in the user configuration file.

commit 18c5c963:

cerbero: Don't warn when there's no user cofiguration
Most people don't have ~/.cerbero/cerbero.cbc, warning about it only
confuses people. Instead, print a message when it *is* used, because
that does indeed cause confusion when a casual user creates it and
then forgets about it.

commit 11d4a302:

cerbero: Use a different prefix and cache for MSVC
When the `visualstudio` variant is enabled, we will use a separate
prefix for dist, sources, logs, and for the cache itself. This avoids
user error while reusing the cerbero prefix.

It will also help with enabling Windows CI on GitLab because the build
cache must not be shared between MinGW and MSVC.