Revert benchmark-workflow pixi PATH workaround

The GPU benchmark runner's pixi 0.69 install resolves on the bare
`pixi` invocations without a $GITHUB_PATH prepend, same as before the
lockfile bump. Restore the workflows to that state.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hmgaudecker

.github/workflows/benchmark-main.yml

@@ -27,8 +27,6 @@ jobs:
       - uses: actions/checkout@v6
         with:
           fetch-depth: 0
-      - name: Put the runner's pixi on PATH
-        run: echo "$HOME/.pixi/bin" >> "$GITHUB_PATH"
       - name: Install environment
         run: pixi install -e benchmarks-cuda12
       - name: Regenerate _version.py

.github/workflows/benchmark-pr.yml

@@ -34,8 +34,6 @@ jobs:
           fetch-depth: 0
       - name: Ensure main ref exists
         run: git fetch origin main:main || true
-      - name: Put the runner's pixi on PATH
-        run: echo "$HOME/.pixi/bin" >> "$GITHUB_PATH"
       - name: Install environment
         run: pixi install -e benchmarks-cuda12
       - name: Regenerate _version.py