Fix broken wireshark build workflow on macOS (apache#318)

Fixes apache#314

We need to run `brew update` to have the fix of
Homebrew/homebrew-core#142254 included.

Author: BewareMyPower

.github/workflows/ci-pr-validation.yaml

@@ -50,6 +50,7 @@ jobs:
       - name: Install deps (macOS)
         if: ${{ startsWith(matrix.os, 'macos') }}
         run: |
+          brew update
           brew install pkg-config wireshark protobuf
       - name: Build wireshark plugin
         run: |