Use correct version of checkout action.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

.github/workflows/checks-basic.yml

diff --git a/.github/workflows/checks-basic.yml b/.github/workflows/checks-basic.yml
index 963eca4..a343a8e 100644
--- a/.github/workflows/checks-basic.yml
+++ b/.github/workflows/checks-basic.yml
@@ -26,7 +26,7 @@
     runs-on: ubuntu-latest
     steps:
 
-    - uses: actions/checkout@main
+    - uses: actions/checkout@v2
       with:
         submodules: true