Disable buffrs::build installing dependencies if we are doing a release build

[mara-schulke]

Or rephrased: Only install missing deps if we are running a debug build and give a nice error hinting on why it is not possible otherwise

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Root cause: When running a release build using cargo in a freshly cloned project, we get an error complaining about buffrs modifing the working dir. This is intended cargo behavior and can be bypassed by --no-verify. In order to give a better error to the user we should not be downloading anything in release builds and error early if dependencies are missing.

In debug builds we want to keep the plug and play nature.