config: add CERBERO_PREFIX_SRC env variable
In the daily usage of Cerbero's shell, it's very convenient have CERBERO_PREFIX_SRC pointing to current sources directory, to jump fast there instead of browsing from CERBERO_PREFIX.
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In the daily usage of Cerbero's shell, it's very convenient have CERBERO_PREFIX_SRC pointing to current sources directory, to jump fast there instead of browsing from CERBERO_PREFIX.