Changes

ci.inria.fr: modernize MinGW builds

* Move ci.inria.fr specific config to the slave profile
  so that we don't explicitly rely on a precis msys2 installation.
* Split 32bits and 64bits builds
  + Check that we're indeed running in the right msys/mingw 32/64 shell.
* Check that we found MSVC lib.exe early.
* Remove many options in the job, they were mostly useful
  when debugging the initial script itself.

New node labels mean we are switching from Win7 to Win10
with a recent MSYS2/MinGW-W64 install.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>