Change get getipaddr to default to loopback

Allows building with no external IP address:

62d0b3d4 introduced a dependency on getipaddr(), which previously raised an error if no
external address was found. This broke bootstrap on a loopback-only network.

closes #5945

base/socket.jl

@@ -564,7 +564,7 @@ function getipaddr()
         end
     end
     ccall(:uv_free_interface_addresses,Void,(Ptr{Uint8},Int32),addr,count)
-    error("no active external interfaces")
+    return ip"127.0.0.1"
 end
 
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