[Enhancement] Revisit GH Action scripts

[glsdown]

Documenting for future PRs because they aren't logically part of this fix

1. The shell script chaining that happens in our workflow steps between getting the file changes and calling sqlfluff lint probably has to go. We've copied it all over the place from our project template and it's not very readable. I'm 90% sure it's been done better in a marketplace action that will save us lines of code
2. More of a style thing. Do we want to combine the workflows into single yaml files based on which events trigger them. This is sort of how GitHub wants you to do it because then you can recycle the name of the status badges e.g."PR Workflows" would have job names "lint" and "test".

Point (1) I feel strongly about point (2) is a take it or leave it type thing but something we should at least look at. We choose between proliferation of yaml files or having long yaml files.

Originally posted by @dluftspring in #314 (review)