Separate prefixes for msvc and mingw inside c:/gstreamer/1.0

commit fb8097e8:

cerbero: Use 'mingw' instead of 'windows' for target_platform
msvc support is no longer 'experimental', and this clarifies that this
target_platform is using mingw both inside Cerbero and inside the CI
YAML.

commit d11bbe00:

Update gitignore to be more specific
And ignore tarballs too.

commit 4a3d7fc1:

cerbero: Don't clobber windows install prefix
We need to ensure that the prefix set by config/win{32,64}.cbc changes
based on the toolchain variant set, otherwise the two will clobber
each other.

I have verified that this does not affect the paths in the MSI
installers.

Requires CI yaml changes: gst-ci!246 (merged)