Validate that a parameterised Slack command matches the corresponding run_args_template

[alarthast]

Fixes #704 .

• Add a validation in build_config that checks that a given parameterised Slack command has parameters that matches the run_args_template for the corresponding job
• Since build_config is run on all our jobs whenever a unit test file imports job_configs, a run of all tests (locally or in CI) will trigger the validation and catch any errors in the raw_config

[rebkwok]

Looks good, thanks for doing this :)