Try env vars for Windows

Author: matthew-brett

.github/workflows/build.ps1

@@ -1,6 +1,6 @@
 echo "::group::Build wheel"
-  pip wheel -w ${WHEEL_SDIR} --no-deps ${REPO_DIR}
-  ls -l "${GITHUB_WORKSPACE}/${WHEEL_SDIR}/"
+  pip wheel -w "${env:WHEEL_SDIR}" --no-deps "${env:REPO_DIR}"
+  ls -l "${env:GITHUB_WORKSPACE}/${env:WHEEL_SDIR}/"
 echo "::endgroup::"
 
 echo "::group::Install wheel"