NSC_SCHEMA does not behave like --schema (local path ignored, unreachable URL silently falls back) #192

NSC_SCHEMA never reaches the schema_override path that --schema uses (nsc/cli/globals.py only passes state.overrides.schema_override). Instead, resolve_profile (nsc/cli/runtime.py) runs it through _url_only and uses it as profile.schema_url. As a result:

• Local path: NSC_SCHEMA=./schema.json nsc dcim sites list ignores the file and fetches from {NSC_URL}/api/schema/.
• Unreachable URL: this goes through the normal fetch, so it silently falls back to the cached or bundled schema. With --schema, it now exits 3 (--schema with a missing file or unreachable URL crashes with an internal error #189/fix(schema): --schema load failures exit 3 instead of crashing (#189) #191).

Expected: NSC_SCHEMA behaves exactly like --schema, with the flag winning when both are set: it accepts a path or URL, never consults the TTL or fallback, and a load failure exits 3 with Error: cannot load schema from <source>: <reason>.

Found while fixing #189.