Explicitly set GITHUB_ACTIONS env var (in cibuildwheel Docker container)

Author: rgommers

.github/workflows/freethreaded_wheels.yml

@@ -69,6 +69,7 @@ jobs:
           CIBW_MANYLINUX_X86_64_IMAGE: manylinux2014
           CIBW_MUSLLINUX_X86_64_IMAGE: musllinux_1_2
           CIBW_BUILD_FRONTEND: "pip; args: --no-build-isolation"
+          GITHUB_ACTIONS: True
       - uses: actions/upload-artifact@65462800fd760344b1a7b4382951275a0abb4808 # v4.3.3
         with:
           name: wheels_linux_${{ matrix.cibw_arch }}_${{ matrix.cibw_python }}