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>