Fix codecov by adding a deeper "fetch-depth" to GitHub Actions

I'm not sure when this became a requirement, but apparently in the
past few days codecov started needing more commit history.

Author: TheBlueMatt

.github/workflows/build.yml

@@ -157,6 +157,8 @@ jobs:
           apt-get -y install cargo valgrind lld git g++ clang
       - name: Checkout source code
         uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
       - name: Sanity test bindings
         working-directory: lightning-c-bindings
         run: cargo check