cmd/cli: support nocgo version for upgrade command

linux/amd64 have the nocgo binary to support system where standard libc
missing.

If the current binary is a nocgo version, "ctrld upgrade" command must
honor the nocgo setting and download the right binary.

cmd/cli/nocgo.go

@@ -0,0 +1,5 @@
+//go:build !cgo
+
+package cli
+
+const cgoEnabled = false