fix: use posix pathing when constructing edge functions

Author: pieh

src/build/functions/edge.ts

@@ -1,5 +1,5 @@
 import { cp, mkdir, readFile, rm, writeFile } from 'node:fs/promises'
-import { dirname, join, relative } from 'node:path'
+import { dirname, join, relative } from 'node:path/posix'
 
 import type { Manifest, ManifestFunction } from '@netlify/edge-functions'
 import { glob } from 'fast-glob'