binutils-testsuite: unset SOURCE_DATE_EPOCH when running the tests

binutils-testsuite: unset SOURCE_DATE_EPOCH when running the tests

The replacing_non_deterministic_member() test in ar.exp explicitly needs
SOURCE_DATE_EPOCH to be unset:

  # This test expects SOURCE_DATE_EPOCH to not be set in the environment

I hope that the test harness could just unset this in advance of running
the tests, but until this is upstreamed we can unset it.

This fixes the test:

  ptestresult.binutils.replacing non-deterministic member (wrong size, expected: 920)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>