Fix actions/checkout@v2 (#1472)

# Changes

Using `actions/checkout@v2` to checkout the github code onto your local
workspace is nice, but can be problematic when using old docker image.
From what I understand, the action uses NodeJS version internally and
therefore requires a modern glibc version as the action can impose newer
NodeJS version and therefore can eventually break our build.

To avoid this, I'm just going to manually checkout the code and run the
submodule update manually. This will fix our C builds.

Author: RReichert

.github/workflows/c.yaml

@@ -31,8 +31,6 @@ jobs:
 
     runs-on: ubuntu-latest
     container: ubuntu:18.04
-    env:
-      ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
 
     steps:
 
@@ -47,10 +45,14 @@ jobs:
           apt-get -qq update
           apt-get -qq install libeigen3-dev libserialport-dev git cmake build-essential ${{ matrix.compiler.package }}
 
-      - uses: actions/checkout@v2
-        with:
-          submodules: recursive
-          fetch-depth: 0
+      - name: Checkout
+        run: |
+          git config --global --add safe.directory "${PWD}"
+          git init
+          git remote add origin "https://github.com/${{ github.repository }}"
+          git fetch --no-tags --depth=1 origin "${{ github.sha }}"
+          git checkout "${{ github.sha }}"
+          git submodule update --recursive --init --depth=1
 
       - name: Configure
         run: |