[node] nvm_install_snippet 'source' should probably be '.'

This line below, produces a `sh: 6: source: not found` error in `/bin/sh`, in contrast to the `.` commands in `nvm_rc_snippet` which work in `/bin/sh` as well as `/bin/bash`:

https://github.com/devcontainers/features/blob/852f1f0567280b465913b487d5f8c773122c36ab/src/node/install.sh#L122

There's a discussion around this error in #440.

Inspect src/node/install.sh at line 122 and compare it with nvm_rc_snippet, especially the command that fails under /bin/sh. Read the discussion in #440 for context and verify the install snippet under /bin/sh; done means it no longer reports the reported shell error.

Written by the indexing model from the issue text.