Fix automatic legacy_cpu detection for polars rustler

[nabijaczleweli]

Before:

$ mix ecto.setup
==> explorer
Compiling 29 files (.ex)

20:07:38.808 [debug] Copying NIF from cache and extracting to /home/nabijaczleweli/work/beambox-logbook/_build/dev/lib/explorer/priv/native/libexplorer-v0.10.1-nif-2.15-x86_64-unknown-linux-gnu.so.tar.gz
Illegal instruction

and indeed I'm missing some features:

$ for f in fxsr sse sse2 ssse3 sse4_1 sse4_2 popcnt avx fma; do grep -m1 flags /proc/cpuinfo  | grep -wq $f || echo no $f; done
no avx
no fma

Some IO.inspects reveal use_legacy is always a bool, since Application.compile_env/3's default argument of System.get_env("EXPLORER_USE_LEGACY_ARTIFACTS") in ["true", "1"] is always a bool:

$ mix deps.get
use_legacy: false
is_nil(use_legacy): false
needed_caps: ["fxsr", "sse", "sse2", "ssse3", "sse4_1", "sse4_2", "popcnt", "avx", "fma"]
Explorer.ComptimeUtils.cpu_with_all_caps?(needed_caps): false
use_legacy or (is_nil(use_legacy) and not Explorer.ComptimeUtils.cpu_with_all_caps?(needed_caps)): false

After:

$ mix ecto.setup
==> explorer
Compiling 29 files (.ex)
use_legacy: nil
is_nil(use_legacy): true
needed_caps: ["fxsr", "sse", "sse2", "ssse3", "sse4_1", "sse4_2", "popcnt", "avx", "fma"]
Explorer.ComptimeUtils.cpu_with_all_caps?(needed_caps): false
if is_nil(use_legacy), do: not Explorer.ComptimeUtils.cpu_with_all_caps?(needed_caps), else: use_legacy: true


20:32:03.923 [debug] Copying NIF from cache and extracting to /home/nabijaczleweli/work/beambox-logbook/_build/dev/lib/explorer/priv/native/libexplorer-v0.10.1-nif-2.15-x86_64-unknown-linux-gnu--legacy_cpu.so.tar.gz