perf session: Add accessor for session->header.env

perf session: Add accessor for session->header.env

The perf_env from the header in the session is frequently accessed,
add an accessor function rather than access directly. Cache the value
to avoid repeated calls. No behavioral change.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250724163302.596743-10-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>