checkout: Fix error on containers

```
git remote add origin https://github.com/taiki-e/cargo-hack
  fatal: detected dubious ownership in repository at '/__w/cargo-hack/cargo-hack'
  To add an exception for this directory, call:

  	git config --global --add safe.directory /__w/cargo-hack/cargo-hack
```

checkout/main.sh

@@ -27,6 +27,9 @@ wd=$(pwd)
 g git version
 
 g git init
+
+g git config --global --add safe.directory "${wd}"
+
 g git remote add origin "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}"
 
 g git config --local gc.auto 0