Scope export to NPR_API_KEY and WRITE_BUCKET

Address PR review: bare 'export' exports every Make variable to
subprocesses; only the credentials sourced from .env need to propagate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: drernie

Makefile

@@ -1,5 +1,5 @@
 sinclude .env # create from example.env
-export
+export NPR_API_KEY WRITE_BUCKET
 PROJECT ?= nf-quilt
 WRITE_BUCKET ?= write-bucket-not-set
 FRAGMENT ?= &path=.