By default a failed binary download falls back to compiling node from source,
which is slow and pointless on platforms that always have prebuilt binaries.
`-b` already disables the fallback per-invocation;
`NVM_NO_SOURCE_FALLBACK=1` makes it the default for every `nvm install`,
so CI images and dev setups need not thread `-b` through every callsite.
It behaves exactly like `-b`
- aborting with a non-zero exit on a failed binary download
- and is mutually exclusive with `-s`, for the same reason `-b` is.
Jordan Harband
9570a22bfe