nvm.sh uses `NVM_SCRIPT_SOURCE="$_"` to detect its source location.
Adding `: nvm.sh` before each source line ensures `$_` is set correctly, preventing breakage when the previous command (e.g., `set -ex`) overwrites it.
Saves us ~25% bandwidth while downloading the payload. This only applies
to hosts that has the `xz` binary and attempts to use iojs 2.3.2 or newer
(this includes nodejs 4.0+ as well). Older targets are unaffected.