Nightly wheels not being build with --enable-nightly

It appears that the nightly wheels are not being built with the --enable-nightly flag, and as such the nightly version is not being added to the version field. It seems that this started happening after 6f5f64fc. After this commit the build added the `--enable-nightly`` via the following:

 - if [[ ${CI_PIPELINE_SOURCE} == schedule ]]; then EXTRA_CONFIG_FLAGS="--enable-nightly"; fi

This is a bash-ism, could that be the problem?