tests/compiance/checkpatch: force the format to 'email'

Otherwise, local settings in the account will cause headache and
randomly alter the result of the testcase.

Change-Id: I758966a7fe9335a13ce237aea7d6df2ddc5733ce
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>