fix: avoid env vars due to dependabot bug

Because:

* Dependabot is running yarn in a manner that breaks the env var replacmeent
  in .yarnrc.yml.

This commit:

* Removes the env var replacement in .yarnrc.yml.

Closes FXA-10002

.yarnrc.yml

@@ -1,6 +1,6 @@
 enableGlobalCache: true
 
-nmMode: ${DC_NMMODE:-hardlinks-global}
+nmMode: hardlinks-global
 
 nodeLinker: node-modules