ci: display more debug information in the init_repo script
I'm *really* confused about the error message [while cloning submodules on Windows on Azure](https://dev.azure.com/rust-lang/e71b0ddf-dd27-435a-873c-e30f86eea377/_apis/build/builds/295/logs/506):
```
/usr/bin/tar: You must specify one of the '-Acdtrux', '--delete' or '--test-label' options
Try '/usr/bin/tar --help' or '/usr/bin/tar --usage' for more information.
```
It doesn't make sense for it to execute a command without any of those flags since they're clearly added:
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||
|---|---|---|
| .. | ||
| docker | ||
| init_repo.sh | ||
| run.sh | ||
| shared.sh | ||