ci: add conditions to rust build code

[lachieh]

Feature or Problem

After adding a path filter to the wasmcloud job, I noticed that PRs with only changes in the washboard-ui directory were stuck in a pending state because of this issue:

If a workflow is skipped due to branch filtering, path filtering, or a commit message, then checks associated with that workflow will remain in a "Pending" state. A pull request that requires those checks to be successful will be blocked from merging.

Solution

This PR adds a new _file-changes.yml workflow which categorizes changed files into groups, and then surfaces those groups as flags if the contents in those files changed.

Note that this is a temporary solution until a repo re-structure takes place and we can consolidate some of the pipeline steps and change the checks that are required for PRs to merge.