perf header: Clean up use of perf_env

Always use the perf_env from the feat_fd's perf_header. Cache the
value on entry to a function in `env` and use `env->` consistently in
the code. Ensure the header is initialized for use in
perf_session__do_write_header.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250724163302.596743-12-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Author: captain5050