ci(build): restore -C target-cpu=x86-64-v3 on published x86_64 builds

Issue #20: !17 (#16) made the x86-64-v3 baseline opt-in via RUSTFLAGS but
nothing set it, so release binaries silently fell back to the plain x86-64
baseline. Set RUSTFLAGS per matrix row on the two x86_64 entries only:

- linux  x86_64-unknown-linux-gnu: -C target-cpu=x86-64-v3
- windows x86_64-pc-windows-msvc: -C target-cpu=x86-64-v3
  -C link-arg=/STACK:8388608 (carries the 8 MB stack link-arg that the
  repo config supplies for Windows, because RUSTFLAGS REPLACES config
  rustflags rather than appending)

The aarch64 rows keep the config's link-args untouched (no RUSTFLAGS at
all), and macOS / android jobs plus .cargo/config.toml are unchanged.

Closes #20

.github/workflows/build.yml

@@ -67,6 +67,15 @@ jobs:
             https://github.com/llvm/llvm-project/releases/download/llvmorg-${{
             env.LLVM_VERSION }}/LLVM-${{ env.LLVM_VERSION }}-Linux-${{
             matrix.llvm }}.tar.xz
+      # The published x86_64 release artifact is the only Linux build that may
+      # use the AVX2/BMI2 baseline (.cargo/config.toml deliberately keeps the
+      # default build portable after !17; see issue #20). RUSTFLAGS REPLACES
+      # config rustflags, so set it here — per matrix row — never in the
+      # config. The aarch64 row must not receive any RUSTFLAGS (an empty
+      # RUSTFLAGS would also drop its -fuse-ld=lld link-arg).
+      - name: Set RUSTFLAGS for x86_64 (x86-64-v3)
+        if: ${{ matrix.target == 'x86_64-unknown-linux-gnu' }}
+        run: echo "RUSTFLAGS=-C target-cpu=x86-64-v3" >> $GITHUB_ENV
       - name: Build
         run: |
           cargo build --profile ${{ env.BUILD_PROFILE }} -v --target=${{ matrix.target }}
@@ -115,6 +124,14 @@ jobs:
             https://github.com/llvm/llvm-project/releases/download/llvmorg-${{
             env.LLVM_VERSION }}/LLVM-${{ env.LLVM_VERSION }}-${{ matrix.llvm
             }}.exe
+      # The published x86_64 Windows artifact must keep the optimised baseline
+      # (issue #20). RUSTFLAGS REPLACES config rustflags, so carry the 8 MB
+      # stack link-arg the config supplies for Windows alongside the CPU flag;
+      # the aarch64 row must not receive any RUSTFLAGS (an empty RUSTFLAGS
+      # would also drop its /STACK:8388608 link-arg).
+      - name: Set RUSTFLAGS for x86_64 (x86-64-v3 + 8 MB stack)
+        if: ${{ matrix.target == 'x86_64-pc-windows-msvc' }}
+        run: echo "RUSTFLAGS=-C target-cpu=x86-64-v3 -C link-arg=/STACK:8388608" >> $env:GITHUB_ENV
       - name: Build
         run: |
           cargo build --profile ${{ env.BUILD_PROFILE }} -v --target=${{ matrix.target }}