install: Fix exec helper to not swallow unexpected args

Our `exec-in-host-mount-namespace` internal helper was intended
to be a "passthrough" that accepts whatever child arguments.

However I misunderstood how to do this in clap.  Before this
change we get:

```
$ bootc blabla
ERROR Re-exec in host mountns: Missing command
```

i.e. the exec code kicks in because `external_subcommand` takes
over *everything* unknown.

Fix things to use `trailing_var_arg` which is intended for this case.

After this patch:

```
$ bootc blabla
error: unrecognized subcommand 'blabla'

Usage: bootc <COMMAND>

For more information, try '--help'.
```

Signed-off-by: Colin Walters <walters@verbum.org>

lib/src/cli.rs

@@ -156,8 +156,10 @@ pub(crate) enum Opt {
     /// Execute the given command in the host mount namespace
     #[cfg(feature = "install")]
     #[clap(hide = true)]
-    #[command(external_subcommand)]
-    ExecInHostMountNamespace(Vec<OsString>),
+    ExecInHostMountNamespace {
+        #[clap(trailing_var_arg = true, allow_hyphen_values = true)]
+        args: Vec<OsString>,
+    },
     /// Install to the target filesystem.
     #[cfg(feature = "install")]
     InstallToFilesystem(crate::install::InstallToFilesystemOpts),
@@ -458,7 +460,7 @@ async fn run_from_opt(opt: Opt) -> Result<()> {
         #[cfg(feature = "install")]
         Opt::InstallToFilesystem(opts) => crate::install::install_to_filesystem(opts).await,
         #[cfg(feature = "install")]
-        Opt::ExecInHostMountNamespace(args) => {
+        Opt::ExecInHostMountNamespace { args } => {
             crate::install::exec_in_host_mountns(args.as_slice())
         }
         Opt::Status(opts) => super::status::status(opts).await,

lib/src/install.rs

@@ -672,7 +672,7 @@ pub(crate) fn run_in_host_mountns(cmd: &str) -> Command {
 
 #[context("Re-exec in host mountns")]
 pub(crate) fn exec_in_host_mountns(args: &[std::ffi::OsString]) -> Result<()> {
-    let (cmd, args) = args[1..]
+    let (cmd, args) = args
         .split_first()
         .ok_or_else(|| anyhow::anyhow!("Missing command"))?;
     let pid1mountns = std::fs::File::open("/proc/1/ns/mnt")?;