vmm: fix tmpfs bind dir empty #1091

Job	Run time
checks (vmm/sandbox, --all-features)	1m 56s
tests (vmm/sandbox, --all-features)	3m 18s
checks (vmm/task, --all-features)	2m 14s
tests (vmm/task, --all-features)	2m 36s
checks (shim, --all-features)	2m 15s
tests (shim, --all-features)	2m 25s
checks (quark, --all-features)	2m 15s
tests (quark, --all-features)	2m 27s
checks (runc, --all-features)	2m 11s
tests (runc, --all-features)	2m 26s
checks (wasm, --features=wasmedge, 0.13.5)	2m 14s
tests (wasm, --features=wasmedge, 0.13.5)	2m 51s
checks (wasm, --features=wasmtime)	2m 14s
tests (wasm, --features=wasmtime)	3m 24s
	34m 46s

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