Merge bitcoin-core/secp256k1#1818: ci: Enforce base-10 evaluation

4fb7ccf5d4 ci: Enforce base-10 evaluation (Hennadii Stepanov)

Pull request description:

  This PR is a follow-up to https://github.com/bitcoin-core/secp256k1/pull/1816. It enforces base-10 evaluation the week number to prevent Bash from interpreting weeks "08" and "09" as invalid octal numbers.

  It was present in my initial [proposal](https://github.com/bitcoin-core/secp256k1/pull/1807#pullrequestreview-3711057184), but somehow I missed it in https://github.com/bitcoin-core/secp256k1/pull/1816.

ACKs for top commit:
  real-or-random:
    ACK 4fb7ccf5d4

Tree-SHA512: dc6d1e5d692ac82616c5c64e1093c19693aa81d3b693834fb0999130b5e3357340b8ed0a5322cfa8d2f9fefbfeb8285abbd580662d257529d9d767afa009e9ef

.github/workflows/ci.yml

@@ -67,7 +67,7 @@ jobs:
     steps:
       - name: Get cache validity period
         id: cache_timestamp
-        run: echo "period=$(($(date +%V) / 4))" >> "$GITHUB_OUTPUT"
+        run: echo "period=$((10#$(date +%V) / 4))" >> "$GITHUB_OUTPUT"
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3