ci: revert to actions/checkout@v1 in manualtest

This enables checkout to work in the Gentoo container.

See: actions/runner#2115

.github/workflows/manualtest.yml

@@ -27,7 +27,7 @@ jobs:
             options: "--privileged -v /dev:/dev"
         steps:
             -   name: "Checkout Repository"
-                uses: actions/checkout@v2
+                uses: actions/checkout@v1
                 with:
                     fetch-depth: 0