github actions: pin checkout action to v1 (#145)

master now contains a new v2 beta version of this action that no longer
supports "submodules".   We can update to v2 later once it stabilizes.

Author: sbc100

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1
       with:
         submodules: true
     - name: Install Rust (rustup)
@@ -32,7 +32,7 @@ jobs:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1
       with:
         submodules: true
     - name: Install Rust