setup: add an escape hatch for "no more default hash algorithm" change

Partially revert c8aed5e8 (repository: stop setting SHA1 as the
default object hash, 2024-05-07), to keep end-user systems still
broken when we have gap in our test coverage but yet give them an
escape hatch to set the GIT_TEST_DEFAULT_HASH_ALGO environment
variable to "sha1" in order to revert to the previous behaviour, in
case we haven't done a thorough job in fixing the fallout from
c8aed5e8.  After we build confidence, we should remove the escape
hatch support, but we are not there yet after only fixing three
commands (hash-object, apply, and patch-id) in this series.

Due to the way the end-user facing GIT_DEFAULT_HASH environment
variable is used in our test suite, we unfortunately cannot reuse it
for this purpose.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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