findRelLink does not work with cached headers

[Fil]

https://github.com/observablehq/oss-analytics/blob/96b2248a435b0e01352eeb5a9cdd78ab1da5258a/src/github.js#L70

this breaks if the headers are taken from the cache; I think the following works in both cases:

-  return headers
-    .get("link")
+  headers = { ...headers };
+  return headers["link"]

[mbostock]

Fixed in 2ed5e5bf96a42a0af869d968e06d9ec057741786.