Skip unknown engine(s) when saving

[muzimuzhi]

This PR solves a l3build save problem that unknown engines passed to -e|--engine are not skipped.

Run texlua ./l3build.lua save -epdftex,XXX 00-test-1 in this repository,

• Before

  Although Skipping unknown engine XXX is printed, unknown engine XXX is not skipped hence the problem sh: XXX: command not found.

  $ texlua ./l3build.lua save -epdftex,XXX 00-test-1
  Skipping unknown engine XXX
  This is pdfTeX, ...
  ...
  Transcript written on 00-test-1.log.
  Creating and copying 00-test-1.XXX.tlg
  sh: XXX: command not found
  ./l3build-check.lua:97: ./build/test/00-test-1.log: No such file or directory

• After, unknown engine is correctly skipped.

  $ texlua ./l3build.lua save -epdftex,XXX 00-test-1
  Skipping unknown engine XXX
  This is pdfTeX, ...
  ...
  Transcript written on 00-test-1.log.