perf evlist: Change env variable to session

perf evlist: Change env variable to session

The session holds a perf_env pointer env. In UI code container_of is
used to turn the env to a session, but this assumes the session
header's env is in use. Rather than a dubious container_of, hold the
session in the evlist and derive the env from the session with
evsel__env, perf_session__env, etc.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250724163302.596743-11-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>