Parameterize fastapi-github example with environment vars (#3078)

## 📝 Summary

Detects `GITHUB_REPO` and `REPO_ROOT` environment variables if set,
falling back to default in marimo.

(I believe this is the desired behavior of the script)

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## 🔍 Description of Changes

Just detects `GITHUB_REPO` and `REPO_ROOT` environment variables if set.

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## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
[Discord](https://marimo.io/discord?ref=pr), or the community
[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [ ] I have added tests for the changes made.
- [ ] I have run the code and verified that it works as expected.

## 📜 Reviewers

@mscolnick

Author: manzt

examples/frameworks/fastapi-github/main.py

@@ -23,8 +23,8 @@
 logger = logging.getLogger(__name__)
 
 # Constants
-GITHUB_REPO = "marimo-team/marimo"
-ROOT_DIR = "examples/ui"
+GITHUB_REPO = os.environ.get("GITHUB_REPO", "marimo-team/marimo")
+ROOT_DIR = os.environ.get("ROOT_DIR", "examples/ui")
 templates_dir = os.path.join(os.path.dirname(__file__), "templates")
 
 # Set up templates