[2023_R2] ci: travis: fix dt-bindings job

[nunojsa]

Make use of pix to install dtschema. This is the preferred way of installing apps from python packages (one cannot use pip anymore in distros like ubuntu 24.04 for example). This creates an isolated virtual environment and ensures the command are accessible in $PATH. It also makes sure all dependencies are met and do not interfere with OS's python packages.

Signed-off-by: Nuno Sa nuno.sa@analog.com
(cherry picked from commit daf69ea)

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