Use checkout action in workflows (#256)

This should simplify things a bit and will hopefully be more robust so we get fewer errors during checkout.

Tested: presubmit (clang-format) and cmake actions still pass. Bazel action is suuuuper slow, so I'm not going to wait for it, but it at least gets to the point of starting the build.
diff --git a/.github/workflows/bazel_build.yml b/.github/workflows/bazel_build.yml
index c58e580..60779d8 100644
--- a/.github/workflows/bazel_build.yml
+++ b/.github/workflows/bazel_build.yml
@@ -47,12 +47,10 @@
           # Update pip before using it so it picks up the latest versions of our dependencies.
           sudo python3 -m pip install --upgrade pip
           sudo python3 -m pip install numpy tf-nightly
-      - name: Checking out latest version and all submodules
-        run: |
-          git config --global remote.origin.fetch '+refs/pull/*:refs/remotes/origin/pull/*'
-          git clone https://github.com/${GITHUB_REPOSITORY?} ${GITHUB_WORKSPACE?} --no-checkout --no-tags
-          git checkout ${GITHUB_SHA?}
-          git submodule update --init --depth 1000 --jobs 8
+      - name: Checking out repository
+        uses: actions/checkout@v2
+      - name: Updating submodules
+        run: git submodule update --init --depth 1000 --jobs 8
       - name: Building and testing with bazel
         run: |
           # Build and test everything not explicitly marked as excluded from CI
diff --git a/.github/workflows/cmake_build.yml b/.github/workflows/cmake_build.yml
index da409ff..1e3796b 100644
--- a/.github/workflows/cmake_build.yml
+++ b/.github/workflows/cmake_build.yml
@@ -33,12 +33,10 @@
           sudo apt-get update
           sudo apt-get install clang libsdl2-dev python3 python3-pip
           sudo pip install numpy
-      - name: Checking out latest version and all submodules
-        run: |
-          git config --global remote.origin.fetch '+refs/pull/*:refs/remotes/origin/pull/*'
-          git clone https://github.com/${GITHUB_REPOSITORY?} ${GITHUB_WORKSPACE?} --no-checkout --no-tags
-          git checkout ${GITHUB_SHA?}
-          git submodule update --init --depth 1000 --jobs 8
+      - name: Checking out repository
+        uses: actions/checkout@v2
+      - name: Updating submodules
+        run: git submodule update --init --depth 1000 --jobs 8
       - name: Building with cmake
         run: |
           mkdir build && cd build
diff --git a/.github/workflows/presubmit.yml b/.github/workflows/presubmit.yml
index 9165cda..ca2140b 100644
--- a/.github/workflows/presubmit.yml
+++ b/.github/workflows/presubmit.yml
@@ -33,11 +33,11 @@
         sudo DEBIAN_FRONTEND=noninteractive apt-get install -qq -y --no-install-recommends clang-format-9
         wget https://raw.githubusercontent.com/llvm-mirror/clang/master/tools/clang-format/git-clang-format -O /tmp/git-clang-format
         chmod +x /tmp/git-clang-format
-    - name: Checking out latest version and all submodules
-      run: |
-        git config --global remote.origin.fetch '+refs/pull/*:refs/remotes/origin/pull/*'
-        git clone https://github.com/$GITHUB_REPOSITORY $GITHUB_WORKSPACE --no-checkout --no-tags
-        git checkout $GITHUB_SHA
+    - name: Checking out repository
+      uses: actions/checkout@v2
+      # We have to explicitly fetch master as well
+    - name: Fetching master
+      run: git fetch --no-tags --prune --depth=1 origin master
     - name: Running clang-format on changed source files
       # TODO(benvanik): actually filter to changed files.
       run: |