Fix time zone for 'cobol.dg/group2/FUNCTION_DATE___TIME_OMNIBUS.cob' [PR119818]

This progresses:

    PASS: cobol.dg/group2/FUNCTION_DATE___TIME_OMNIBUS.cob   -O0  (test for excess errors)
    [-FAIL:-]{+PASS:+} cobol.dg/group2/FUNCTION_DATE___TIME_OMNIBUS.cob   -O0  execution test
    [Etc.]

	PR cobol/119818
	gcc/testsuite/
	* cobol.dg/group2/FUNCTION_DATE___TIME_OMNIBUS.cob:
	'dg-set-target-env-var TZ UTC0'.

(cherry picked from commit ed8761241ac529ccddb2b76a1895c124c67c132c)