ci: run test round with parallel-checkout enabled

We already have tests for the basic parallel-checkout operations. But
this code can also run be executed by other commands, such as
git-read-tree and git-sparse-checkout, which are currently not tested
with multiple workers. To promote a wider test coverage without
duplicating tests:

1. Add the GIT_TEST_CHECKOUT_WORKERS environment variable, to optionally
   force parallel-checkout execution during the whole test suite.

2. Set this variable (with a value of 2) in the second test round of our
   linux-gcc CI job. This round runs `make test` again with some
   optional GIT_TEST_* variables enabled, so there is no additional
   overhead in exercising the parallel-checkout code here.

Note that tests checking out less than two parallel-eligible entries
will fall back to the sequential mode. Nevertheless, it's still a good
exercise for the parallel-checkout framework as the fallback codepath
also writes the queued entries using the parallel-checkout functions
(only without spawning any worker).

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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