ci(verify-proofs): mkdir extracted/ before make (gitignored output dir)

extraction.v's [Extraction "extracted/filesystem.ml" ...] writes into the
gitignored directory, which doesn't exist on a fresh CI clone. Add a one-
line mkdir before the make step.

Local fresh clone reproduces the issue without this fix.

Author: hyperpolymath

.github/workflows/validation.yml

@@ -62,6 +62,8 @@ jobs:
       - name: Build Coq proofs (oracle)
         working-directory: proofs/coq
         run: |
+          # extraction.v writes to extracted/filesystem.ml; the dir is gitignored, so create it.
+          mkdir -p extracted
           coq_makefile -f _CoqProject -o Makefile
           make -j1