testsuite, jit: fix test-error-pr63969-missing-driver.c

jit.dg/test-error-pr63969-missing-driver.c tries to break PATH and
verify that an error is generated when using an external driver.

However it does this by unsetting PATH, and so the test could
accidentally find the driver if the system supplies a default and the
driver happens to be installed in that path (reported as rhbz#2318021).

Fix the test by instead setting PATH to a bogus value.

gcc/testsuite/ChangeLog:
	* jit.dg/test-error-pr63969-missing-driver.c (create_code): When
	breaking PATH, use setenv with a bogus value, rather than
	unsetenv, in case the system uses a default path that contains
	the driver binary.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>