perf test: Replace legacy `...` with $(...)

perf test: Replace legacy `...` with $(...)

As detailed in https://www.shellcheck.net/wiki/SC2006:

The use of `...` is legacy syntax with several issues:
1. It has a series of undefined behaviors related to quoting in POSIX.
2. It imposes a custom escaping mode with surprising results.
3. It's exceptionally hard to nest.

$(...) command substitution has none of these problems,
and is therefore strongly encouraged.

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>
Acked-by: Carsten Haitzler <carsten.haitzler@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230201214945.127474-3-didi.debian@cknow.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>