tc-testing: fix eBPF tests failure on linux fresh clones

when the following command is done on a fresh clone of the kernel tree,

 [root@f31 tc-testing]# ./tdc.py -c bpf

test cases that need to build the eBPF sample program fail systematically,
because 'buildebpfPlugin' is unable to install the kernel headers (i.e, the
'khdr' target fails). Pass the correct environment to 'make', in place of
ENVIR, to allow running these tests.

Fixes: 4c2d39bd ("tc-testing: use a plugin to build eBPF program")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>