fix: prevent layer overwrites in image resulting in BLOB_UNKNOWN error

[mafredri]

This is most likely a bug in https://github.com/google/go-containerregistry, but we can work around it in Kaniko. In fact, the only bug in go-containerregistry may be that this behavior is allowed.

// Images in google/go-containerregistry don't support adding unique layers
// with duplicate diff IDs. For example, if the source image has an empty
// layer which has been compressed with Gzip level 3, and the layer we're
// adding is also an empty layer compressed with Gzip level 1, the diff IDs
// would match but the layer blobs would be different. This would cause an
// error when trying to upload the image to a registry as the manifest is
// referencing a blob that has been "overwritten".

Refs coder/envbuilder#385
Closes #34

[johnstcn]

🎉