secretsprovider.NewStore() simplify env handling
secretsprovider.NewStore() simplify env handling

The `doesEnvExist()` utility was copied from the classic builder,
and was developed when `os.LookupEnv()` did not yet exist.

Now that it's available, replace our custom implementation in
favor of Golang's function.

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc