.buildkite: run.sh: Install lpc_checksum package

The sanitycheck tests are all failing for the lpcxpresso11u68 board
because the lpc_checksum, a new python script requirement introduced
by this board, is not present in the build environment. This
commit attempt to fix this issue by installing lpc_checksum before
running sanitycheck.

Signed-off-by: Maxime Bittan <maxime.bittan@seagate.com>