b2/s3: Move config validation from ApplyEnvironment to Open/Create

Conceptually the backend configuration should be validated when creating
or opening the backend, but not when filling in information from
environment variables into the configuration.

internal/restic/backend.go

@@ -83,5 +83,5 @@ type FileInfo struct {
 
 // ApplyEnvironmenter fills in a backend configuration from the environment
 type ApplyEnvironmenter interface {
-	ApplyEnvironment(prefix string) error
+	ApplyEnvironment(prefix string)
 }