ci: Use checkout@v1 action

The checkout@v2 action currently has a bug that makes it sometimes
check out the wrong commit. Since we can't have that we'll use v1 for
now, until that is fixed.

Author: teskje

.github/workflows/ci.yml

@@ -1,3 +1,7 @@
+# Note: The checkout@v2 action sometimes checks out the wrong commit:
+#   https://github.com/actions/checkout/issues/237
+# Until that is fixed we use checkout@v1 here instead.
+
 name: CI
 
 on:
@@ -35,7 +39,7 @@ jobs:
           - mcu: stm32f303xc
             features: rt,stm32-usbd
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
@@ -51,7 +55,7 @@ jobs:
     name: Clippy
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: nightly
@@ -70,7 +74,7 @@ jobs:
     name: Rustfmt
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v1
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable