github/workflows: Fix code-size CI workflow.

Changes are:
- Use ubuntu-20.04 so that gcc-multilib installs without error.
- Use "fetch-depth: 100" to get history prior to pull request.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/code_size.yml

@@ -14,9 +14,11 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: 100
     - name: Install packages
       run: source tools/ci.sh && ci_code_size_setup
     - name: Build