fixup! pythongh-127432: Add CI job to cross build Python

Author: matthewhughes934

.github/workflows/build.yml

@@ -528,7 +528,7 @@ jobs:
     steps:
       - name: Set build dir
         run:
-          echo "$BUILD_DIR=$(realpath ${{ github.workspace }}/../build)" >> "$GITHUB_ENV"
+          echo "BUILD_DIR=$(realpath ${{ github.workspace }}/../build)" >> "$GITHUB_ENV"
       - uses: actions/checkout@v4
         with:
           persist-credentials: false