Fix path handling in scripts/build-challenge-map-graphql.mjs to support Windows (double C:/ bug)

[NewtonLC]

Problem:
When running node scripts/build-challenge-map-graphql.mjs on Windows, the script fails with an error:

Error building challenge map: Error: ENOENT: no such file or directory, mkdir 'C:\C:\Users\newto\dev\classroom\data'
    at async mkdir (node:internal/fs/promises:859:10)
    at async buildChallengeMapFromGraphQL (file:///C:/Users/newto/dev/classroom/scripts/build-challenge-map-graphql.mjs:161:5) {
  errno: -4058,
  code: 'ENOENT',
  syscall: 'mkdir',
  path: 'C:\\C:\\Users\\newto\\dev\\classroom\\data'
}

The script is incorrectly generating a path with two C:/ prefixes (e.g., C:\C:\Users...).

Expected Behavior:

• The script should correctly resolve file paths on all platforms (Windows, macOS, Linux).
• Directory creation and path handling should work regardless of the user's operating system, without duplicating path roots.

Steps to Reproduce:

1. On a Windows machine, run node scripts/build-challenge-map-graphql.mjs.
2. Observe the ENOENT error with the double C:/ path.

Suggested Solution:

• Use Node.js's built-in path module (path.join/path.resolve) instead of manual string concatenation to construct file paths.
• Audit the script for any locations where paths are manually combined, and replace them with cross-platform-safe path handling.

Relevant File:

• scripts/build-challenge-map-graphql.mjs