CUDA single lock

Circumvent issues with https://github.com/prefix-dev/pixi/issues/1051 and declare a single manifest that also contains CUDA environment support.

doc/install.rst

@@ -58,15 +58,12 @@ Choose the relevant tab for details on each of those installation paths.
    .. code:: sh
 
       git clone git@gitlab.idiap.ch:biosignal/software/mednet
-      cd helpers/cuda
-      pixi install --frozen
-      pixi run mednet info
-
-   .. tip::
+      pixi install --frozen --environment cuda
+      pixi run -e cuda mednet info
 
-      The ``--frozen`` flag will ensure that the latest lock-file available
-      with sources is used.  If you'd like to update the lock-file to the
-      latest set of compatible dependencies, remove that option.
+   To enable CUDA support, always run applications using the ``cuda``
+   environment via ``pixi run -e cuda ...``.  Refer to further pixi
+   configuration tips on the *Development* tab above.
 
 
 .. _mednet.setup: