When the run-time tests to build rust and rust-bin packages are run via Docker,
the $USER environment variable is not set, which makes cargo fail when
initializing the test project.
So add it to make cargo happy.
Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>