- Switch to node 14.x instead of 18.x, because the latter doesn't seem
to be compatible with ubuntu 18.x:
`node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by node)`

- Also, remove the existing `nodejs` package before installing a new
one, because these machines are shared, and would get affected by
installations performed by other runs.

…and set _commandExitCode

This allows cases which don't care about this, to continue working,
because they won't set `$PERF_PREREQS_INSTALL_FAILED`, and default to
continuing with the job.