Update all CI workflows to ignore changes to other CI workflows

For example, AppVeyor now ignores changes that only affect other CI systems:

https://github.com/boostorg/boost-ci/commit/aa7946ba5a5dcc2a6599a8a30fcd8e1c10359f0d#diff-e1ab4c3cec24f841948eb867754d4ad0b9b6d11eaa6b5bfbe0357aceb0a49effR28-R44