fix(vm): a single quoted payload argument runs through the image's shell

'ecrou run alpine -- "apk add x && y"' reached the guest as one argv
element and execvp'd a program named after the whole line. One
argument containing whitespace is a quoted shell line: hand it to the
image's shell as 'sh -c'. Argv vectors stay literal, including ones
that begin with sh -c.

crates/ecrou-cli/src/cli.rs

@@ -29,7 +29,9 @@ pub enum Command {
 pub struct RunArgs {
     /// Distribution name or OCI image reference
     pub image: String,
-    /// Command to run in the machine; the image's default shell when omitted
+    /// Command to run in the machine; the image's default shell when
+    /// omitted. An argument vector after `--` is taken literally; a
+    /// single quoted line runs through the image's shell.
     #[arg(last = true)]
     pub command: Vec<String>,
     /// Directory to share into the machine as HOST:GUEST[:ro] (repeatable)