Fix automatic legacy_cpu detection for polars rustler

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

	$ 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

	$ 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

Sponsored-by: https://beaverlabs.net

Author: nabijaczleweli

lib/explorer/polars_backend/native.ex

@@ -8,22 +8,27 @@ defmodule Explorer.PolarsBackend.Native do
   # We want "debug" in dev and test because it's faster to compile.
   mode = if Mix.env() in [:dev, :test], do: :debug, else: :release
 
-  use_legacy =
+  use_legacy = (
+    env = System.get_env("EXPLORER_USE_LEGACY_ARTIFACTS")
+
     Application.compile_env(
       :explorer,
       :use_legacy_artifacts,
-      System.get_env("EXPLORER_USE_LEGACY_ARTIFACTS") in ["true", "1"]
+      (if is_nil(env), do: nil, else: env in ["true", "1"])
     )
+  )
 
   variants_for_linux = [
     legacy_cpu: fn ->
       # These are the same from the release workflow.
       # See the meaning in: https://unix.stackexchange.com/a/43540
       needed_caps = ~w[fxsr sse sse2 ssse3 sse4_1 sse4_2 popcnt avx fma]
 
-      use_legacy or
-        (is_nil(use_legacy) and
-           not Explorer.ComptimeUtils.cpu_with_all_caps?(needed_caps))
+      if is_nil(use_legacy) do
+        not Explorer.ComptimeUtils.cpu_with_all_caps?(needed_caps)
+      else
+        use_legacy
+      end
     end
   ]