t1016: make sure to use specified GPG

c348192a (t1016: clean up style, 2024-10-22) fixed a coding style violation that has an extra space between redirection operator ">" and the redirection target, but at the same time, replaced the use of "git config" to set a configuration variable to be used by the remainder of tests with "test_config". The pattern employed here is that the first set-up test prepares the environment to be used by subsequent tests, which then use the settings left by this set-up test to perform their tasks. Using test_config in the first set-up test means the config setting made by the set-up test is reverted at the end of the first set-up test, which totally misses the point. Go back to use "git config" to fix this. Signed-off-by: Junio C Hamano <gitster@pobox.com>