perf-lib.sh: forbid the use of GIT_TEST_INSTALLED

perf-lib.sh: forbid the use of GIT_TEST_INSTALLED

As noted in preceding commits setting GIT_TEST_INSTALLED has never
been supported or documented, and as noted in an earlier t/perf/README
change to the extent that it's been documented nobody's notices that
the example hasn't worked since 3c8f12c96c ("test-lib: reorder and
include GIT-BUILD-OPTIONS a lot earlier", 2012-06-24).

We could directly support GIT_TEST_INSTALLED for invocations without
the "run" script, such as:

    GIT_TEST_INSTALLED=../../ ./p0000-perf-lib-sanity.sh
    GIT_TEST_INSTALLED=/home/avar/g/git ./p0000-perf-lib-sanity.sh

But while not having this "error" will "work", it won't write the the
resulting "test-results/*" files to the right place, and thus a
subsequent call to aggregate.perl won't work as expected.

Let's just tell the user that they need to use the "run" script,
which'll correctly deal with this and set the right
PERF_RESULTS_PREFIX.

If someone's in desperate need of bypassing "run" for whatever reason
they can trivially do so by setting "PERF_SET_GIT_TEST_INSTALLED", but
not we won't have people who expect GIT_TEST_INSTALLED to just work
wondering why their aggregation doesn't work, even though they're
running the right "git".

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