network: fix private sandbox netns #912

Job	Run time
tests (vmm/sandbox, --all-features)	3m 18s
checks (vmm/sandbox, --all-features)	4m 42s
tests (vmm/task, --all-features)	2m 32s
checks (vmm/task, --all-features)	4m 6s
tests (shim, --all-features)	2m 28s
checks (shim, --all-features)	3m 56s
tests (quark, --all-features)	2m 26s
checks (quark, --all-features)	3m 48s
tests (runc, --all-features)	2m 39s
checks (runc, --all-features)	3m 53s
tests (wasm, --features=wasmedge, 0.11.2)	2m 51s
checks (wasm, --features=wasmedge, 0.11.2)	4m 9s
tests (wasm, --features=wasmtime)	3m 11s
checks (wasm, --features=wasmtime)	4m 14s
	48m 13s

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