Organize and feature-gate FASTSim-2 compatibility code

.github/workflows/build_and_test.yaml

@@ -34,18 +34,20 @@ jobs:
               version: latest
             ls: dir
     runs-on: ${{ format('{0}-{1}', matrix.os.base, matrix.os.version) }}
-    defaults:
-      run:
-        shell: pixi run -e wheels bash -e {0}
     env:
       CARGO_HTTP_MULTIPLEXING: "false"
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Pixi
-        uses: prefix-dev/setup-pixi@v0.8.14
+      - name: Set up Python
+        uses: actions/setup-python@v5
         with:
-          environments: wheels
+          python-version: "3.14"
+
+      - name: Install wheel build dependencies
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install --group wheels
 
       - name: Check version consistency
         if: matrix.os.base == 'ubuntu'
@@ -65,6 +67,7 @@ jobs:
 
       - name: Run Rust tests
         if: matrix.os.base == 'ubuntu'
+        shell: bash -e {0}
         run: cargo test --release
 
       - name: Build source distribution
@@ -82,7 +85,7 @@ jobs:
       # One invocation per platform builds every target Python version --
       # cibuildwheel manages fetching/using each target interpreter internally.
       - name: Build ${{ matrix.platform || matrix.os.base }} binaries
-        run: cibuildwheel --output-dir dist
+        run: python -m cibuildwheel --output-dir dist
         env:
           # Python version targets
           CIBW_BUILD: "cp310-* cp311-* cp312-* cp313-* cp314-*"
@@ -98,7 +101,7 @@ jobs:
           # Linux builds run in an isolated container with no Rust preinstalled,
           # so it needs its own toolchain install.
           CIBW_BEFORE_BUILD_LINUX: >
-            yum -y install openssl openssl-devel &&
+            yum -y install openssl openssl-devel perl perl-IPC-Cmd &&
             curl https://sh.rustup.rs -sSf | sh -s -- --profile=minimal -y &&
             rustup show