perf top: Make perf_env locally scoped

The use of the global host perf_env variable is potentially
inconsistent within the code. Switch perf top to using a locally
scoped variable that is generally accessed through the session.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250724163302.596743-15-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>