Bump actions/checkout from 2 to 2.3.4

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 2.3.4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2...v2.3.4)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         board: [pi2, pi3-64, pi4-64]
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
       with:
         submodules: recursive
 
@@ -67,7 +67,7 @@ jobs:
       boot-home: external/circle-stdlib/libs/circle/boot
 
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.4
       with:
         submodules: recursive