perf evsel: Fall back to global 'perf_env' in perf_evsel__env()

I.e. if evsel->evlist or evsel->evlist->env isn't set, return the
environment for the running machine, as that would be set if reading
from a perf.data file.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-uqq4grmhbi12rwb0lfpo6lfu@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>