CI: Add license-check job

This adds a CI job that uses Reuse Software's lint tool to make sure that
any newly added files have a license and copyright header.
15 jobs for copyright-conformance in 4 minutes and 51 seconds (queued for 1 second)
Status Job ID Name Coverage
  Source Check
passed #999320
build_recommended

00:01:08

passed #999321
eslint

00:00:29

passed #999322
iwyu

00:00:22

passed #999323
license-check

00:00:18

 
  Test
passed #999325
build_maximal

00:03:42

passed #999326
build_minimal

00:00:46

passed #999324
asan
sanitizer_gcc

00:01:13

 
  Manual
manual #999327
allowed to fail manual
coverage
manual #999330
allowed to fail manual
installed_tests
manual #999328
allowed to fail manual
iwyu-full
manual #999329
asan allowed to fail manual
sanitizer_clang
manual #999331
allowed to fail manual
valgrind
manual #999334
allowed to fail manual
zeal_11
manual #999332
allowed to fail manual
zeal_2
manual #999333
allowed to fail manual
zeal_4