t/perf/run: fix bin-wrappers computation

t/perf/run: fix bin-wrappers computation

The GIT_TEST_INSTALLED was moved from perf-lib.sh to run in df0f5021
(perf-lib.sh: remove GIT_TEST_INSTALLED from perf-lib.sh, 2019-05-07)
and that included a change to how it inspected the existence of a
bin-wrappers directory. However, that included a typo that made the
match of bin-wrappers never work. Specifically, the assignment was

	mydir_abs_wrappers="$mydir_abs_wrappers/bin-wrappers"

which uses the same variable before it is initialized. By changing it to

	mydir_abs_wrappers="$mydir_abs/bin-wrappers"

We can correctly use the bin-wrappers directory.

This is critical to successfully computing performance of commands that
execute subcommands. The bin-wrappers ensure that the --exec-path is set
correctly.

Reported-by: Victoria Dye <vdye@github.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>