Currently automatically setting the node version (by setting the
`KEYMAN_USE_NVM` env variable) is kind of broken on Linux because it
still requires to put `~/.keyman/node` on the PATH. This basically
limits us to the node version Keyman needs.
This change checks the path variable and adds `~/.keyman/node` if
necessary. This allows to not have that on the path, thus allowing to
use different node versions outside of Keyman, but still allows Keyman
to build with the node version it needs.
Test-bot: skip
Consolidates the node-related script functions into node.inc.sh, as part
of cleaning up the build scripts and making them easier to maintain into
the future.
Fixes: #14447