sensor: Fix up target app HOME env var when run as user

[iximiuz]

Sensor typically needs to run as root while the target app may need to use a less privileged user. By default, conainer runtimes (like Docker) set the HOME env var upon container startup based on the container's user and the corresponding record in the /etc/passwd file in the image (if any, "/" otherwser). However, instrumented container's user is often different from the target app's user. Thus, sensor needs extra logic to restore the right HOME var doing similar computations.

NB: Having HOME env var set is mandatory in accordance with POSIX.

[ghost]

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