init: do not chdir into initfs prefix after switch_root

The restored switch_root chdir set init's CWD to /scheme/initfs on the
live image. login later restricts its namespace to DEFAULT_SCHEMES (no
'initfs' scheme) via mkns/setns; the inherited /scheme/initfs CWD fd then
becomes unresolvable. Because relibc resolves paths through the CWD fd
(AT_REDOX_CWD_FD), every subsequent file op in login hangs -- notably
Command::spawn() loading the login shell binary never returns, so the
shell never starts. Leaving the CWD inherited (as before the chdir was
added) keeps spawn working across the login namespace switch; getcwd() in
/usr binaries works without the chdir. Reverts the CWD half of 521c6ffe;
the init-diag cleanup from that commit is retained.

init/src/main.rs

@@ -54,21 +54,15 @@ impl InitConfig {
 fn switch_root(unit_store: &mut UnitStore, config: &mut InitConfig, prefix: &Path, etcdir: &Path) {
     status_ok(&format!("switchroot to {} {}", prefix.display(), etcdir.display()));
 
-    // Point init's working directory at a valid directory in the new root so
-    // daemons spawned afterwards don't inherit an invalid CWD (via
-    // AT_REDOX_CWD_FD). Without this, getcwd() fails in /usr binaries — e.g.
-    // the 00_base-env `zsh -c` setup printed "zsh: Failed to get current
-    // directory: path invalid" and then hung, so init (which waits on that
-    // oneshot) never reached the login stack. The switch is logical (no
-    // chroot) and `prefix` (e.g. "/usr") is not itself a directory, so try the
-    // plausible roots in order and keep the first that works.
-    let cwd_candidates = [prefix.to_path_buf(), std::path::PathBuf::from("/")];
-    if !cwd_candidates
-        .iter()
-        .any(|cand| std::env::set_current_dir(cand).is_ok())
-    {
-        init_warn("failed to establish a valid working directory after switch_root");
-    }
+    // NOTE: Do NOT chdir into `prefix` here. On the live image `prefix` is
+    // `/scheme/initfs`, and `login` later restricts its namespace to
+    // DEFAULT_SCHEMES (which does not include `initfs`) via mkns/setns. Any CWD
+    // inherited from init that lives on a scheme absent from that restricted
+    // namespace becomes unresolvable, and because relibc resolves paths through
+    // the CWD fd (AT_REDOX_CWD_FD), *all* subsequent file operations in login —
+    // including `Command::spawn()` loading the shell binary — then hang. Leaving
+    // the CWD as inherited keeps process spawning working after the login
+    // namespace switch. (getcwd() in /usr binaries works without this chdir.)
 
     config
         .envs