de-duplicate `-f <pc.yaml>` flags to avoid the issue in #2700

[jay-aye-see-kay]

Summary

Using the --process-compose-file flag to merge configurations fails with FTL error="project /Users/jack/code/my-porject/process-compose.yaml is already specified in files to load"

See #2700 for details

How was it tested?

Using the method described here https://github.com/jetify-com/devbox/issues/2699 to build and "install" a local devbox then:

1. Create a process-compose.yaml file
2. Create a second process-compose.extras.yaml file that overrides some of the config defined in the main one (i.e set is_disabled: "false")
3. run devbox services up --process-compose-file process-compose.extras.yaml and see error message above instead of running services

The above fails with devbox 0.16.0 but works fine with this PR.

Notes

I'm happy with this fix but not certain this is the best place to implement this fix, it feels like a bit of a bandaid rather than fixing it at the source. But reading the code I couldn't find the root cause.

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers under the terms of the Apache 2 License.

By creating this pull request, I represent that I have the right to license the contributions to the project maintainers under the Apache 2 License as stated in the Community Contribution License.

Closes #2700