Shell-quote values interpolated into Buildkite pipeline commands (#2824)

Follow-up to #2666: apply `shlex.quote` consistently to the remaining
pipeline-command builders so every value interpolated into a
`project_pipeline` command line is quoted the same way.

- `create_config_validation_steps`: quote `--file_config` (the config
file name comes from the PR's modified files)
- `upload_project_pipeline_step`: quote `project_name`,
`git_repository`, `http_config`, `file_config` and `git_commit` instead
of manual double quotes / raw interpolation

Updates the corresponding test expectation to the `shlex.quote` style.