perf README: correct docs for 3c8f12c96c regression

perf README: correct docs for 3c8f12c96c regression

Since 3c8f12c96c ("test-lib: reorder and include GIT-BUILD-OPTIONS a
lot earlier", 2012-06-24) the suggested advice of overriding
GIT_BUILD_DIR has not worked. We've printed a hard error like this
given e.g. GIT_BUILD_DIR=/home/avar/g/git:

    /bin-wrappers/git is not executable; using GIT_EXEC_PATH
    error: You haven't built things yet, have you?

Let's just suggest that the user run other gits via the "run"
script. That'll do the right thing for setting the path to the other
git, and running the "aggregate.perl" scripts afterwards will work.

As an aside, if setting GIT_BUILD_DIR had still worked, then the
MODERN_GIT feature/fix added in 1a0962dee5 ("t/perf: fix regression in
testing older versions of git", 2016-06-22) would have broke.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>