MNT: doc/build.sh: Fix crash when run locally

Author: kernc

doc/build.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -eu
-IS_RELEASE="$([[ "$GITHUB_REF" == refs/tags/* ]] && echo 1 || true)"
+IS_RELEASE="$([[ "${GITHUB_REF:-}" == refs/tags/* ]] && echo 1 || true)"
 
 die () { echo "ERROR: $*" >&2; exit 2; }