fix(ci): downgrade [email protected] back to 2.6.12 (#1877)

This could explain why we are seeing `write EPIPE` errors in the `release_publish_dry_run`
task with Octokit (esp. considering node-fetch/node-fetch@65ae25a1da2834b).
The 2.x line of node-fetch is only used in development dependencies, so let's try this
downgrade for now.

Author: addaleax

package-lock.json

@@ -70,7 +70,7 @@
     "download": "^8.0.0",
     "es-aggregate-error": "^1.0.9",
     "nock": "^13.0.11",
-    "node-fetch": "^2.6.1",
+    "node-fetch": "2.6.12",
     "pkg-up": "^3.1.0",
     "rimraf": "^3.0.2",
     "semver": "^7.5.3",