Store repos under repo name so you can build multiple

[zmrow]

Description of changes:
While testing #1133 we found that building multiple repos defined in Infra.toml wouldn't work because the path didn't include the repo name, causing a conflict. You had to delete directories from previous repo builds even though they're unrelated. This change separates created repos using their names.

Testing done:
Built 2 repos with different names and saw the latest links pointing the

build/repos/
├── default
│   ├── bottlerocket-1.0.1-dafe3b16-dirty
│   │   ├── aws-k8s-1.17
...
│   └── latest -> bottlerocket-1.0.1-dafe3b16-dirty
└── default2
    ├── bottlerocket-1.0.1-dafe3b16-dirty
    │   ├── aws-k8s-1.17
...
    └── latest -> bottlerocket-1.0.1-dafe3b16-dirty

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.