chore: Fix full clean instructions in CONTRIBUTING.md (#26915)

Fix full clean instructions in CONTRIBUTING.md - clean build requires a yarn install first.

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Author: mikewrighton

CONTRIBUTING.md

@@ -1139,6 +1139,7 @@ Most build issues can be solved by doing a full clean rebuild:
 
 ```shell
 $ git clean -fqdx .
+$ yarn install
 $ yarn build
 ```