rustup-init.sh: Fix unset variable usage
With `set -u` our check for `RUSTUP_UPDATE_ROOT` being empty
needed an additional tweak for it being unset.
set -u
-if [ -z "$RUSTUP_UPDATE_ROOT" ]; then
- RUSTUP_UPDATE_ROOT="https://static.rust-lang.org/rustup"
-fi
+# If RUSTUP_UPDATE_ROOT is unset or empty, default it.
+RUSTUP_UPDATE_ROOT="${RUSTUP_UPDATE_ROOT:-https://static.rust-lang.org/rustup}"
#XXX: If you change anything here, please make the same changes in setup_mode.rs
usage() {
