weldr: allow running on unsupported distros

When the host distribution is not known or supported, instead of failing
with an error, print a warning to the log and initialise the API with
the architecture name and distro name.
This enables running the weldr API on unsupported distros for
cross-distro building.

Guards against a nil arch member when initialising the store.

internal/store/json.go

@@ -378,6 +378,9 @@ func imageTypeToCompatString(imgType distro.ImageType) string {
 }
 
 func imageTypeFromCompatString(input string, arch distro.Arch) distro.ImageType {
+	if arch == nil {
+		return nil
+	}
 	for k, v := range imageTypeCompatMapping {
 		if v == input {
 			imgType, err := arch.GetImageType(k)