t/perf/run: preserve GIT_PERF_* from environment

If you run:

  GIT_PERF_LARGE_REPO=/some/path ./p1006-cat-file.sh

it will use the repo in /some/path. But if you use the "run" helper
script to aggregate and compare results, like this:

  GIT_PERF_LARGE_REPO=/some/path ./run HEAD^ HEAD p1006-cat-file.sh

it will ignore that variable. This is because the presence of the
LARGE_REPO variable in GIT-BUILD-OPTIONS overrides what's in the
environment. This started with 4638e8806e (Makefile: use common template
for GIT-BUILD-OPTIONS, 2024-12-06), which now writes even empty
variables (though arguably it was wrong even before with a non-empty
value, as we generally prefer the environment to take precedence over
on-disk config).

We had the same problem in perf-lib.sh itself, and we hacked around it
with 32b74b9809 (perf: do allow `GIT_PERF_*` to be overridden again,
2025-04-04). That's what lets the direct invocation of "./p1006" work
above.

And in fact that was sufficient for "./run", too, until it started
loading GIT-BUILD-OPTIONS itself in 5756ccd181 (t/perf: fix benchmarks
with out-of-tree builds, 2025-04-28). Now it has the same problem: it
clobbers any incoming GIT_PERF options from the environment.

We can use the same hack here in the "run" script. It's quite ugly, but
it's just short enough that I don't think it's worth trying to factor it
out into a common shell library.

In the long run, we might consider teaching GIT-BUILD-OPTIONS to be more
gentle in overwriting existing entries. There are probably other
GIT_TEST_* variables which would need the same treatment. And if and
when we come up with a more complete solution, we can use it in both
spots.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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