spindle: mount /tmp with `exec`

go test runs binaries from /tmp directory

Signed-off-by: oppiliappan <[e-mail]>

spindle/engine/engine.go

···
 				ReadOnly: false,
 				TmpfsOptions: &mount.TmpfsOptions{
 					Mode: 0o1777, // world-writeable sticky bit
+					Options: [][]string{
+						{"exec"},
+					},
 				},
 			},
 			{