Dynamically detect AVX512 in CI

We would like to check for errors with AVX512,
but we don't pick our CPU. So, detect available features.
This variance in checks stochastically reveals issues.
Nondeterminism is acceptable as our goal is protecting downstream.

.github/workflows/ci.yml

@@ -228,14 +228,14 @@ jobs:
         run: cross test --verbose --target=${{ matrix.target }} --release
 
   features:
-    name: "Check cargo features (${{ matrix.features }} ${{ matrix.rustflags }})"
+    name: "Check cargo features (${{ matrix.simd }} × ${{ matrix.features }})"
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix:
-        rustflags:
+        simd:
           - ""
-          - "-Ctarget-feature=+avx512f" # AVX-512 uses packed bit masks, so enable it to test more code paths
+          - "avx512"
         features:
           - ""
           - "--features std"
@@ -248,7 +248,13 @@ jobs:
         run: |
           rustup update nightly --no-self-update
           rustup default nightly
+      - name: Detect AVX512
+        run: echo "CPU_FEATURE=$(lscpu | grep -o avx512[a-z]* | sed s/avx/+avx/ | tr '\n' ',' )" >> $GITHUB_ENV
       - name: Check build
-        run: cargo check --all-targets --no-default-features ${{ matrix.features }}
-        env:
-            RUSTFLAGS: -Dwarnings ${{ matrix.rustflags }}
+        if: ${{ matrix.simd == '' }}
+        run: RUSTFLAGS="-Dwarnings" cargo check --all-targets --no-default-features ${{ matrix.features }}
+      - name: Check AVX
+        if: ${{ matrix.simd == 'avx512' && contains(env.CPU_FEATURE, 'avx512') }}
+        run: |
+          echo "Found AVX features: $CPU_FEATURE"
+          RUSTFLAGS="-Dwarnings -Ctarget-feature=$CPU_FEATURE" cargo check --all-targets --no-default-features ${{ matrix.features }}