deb: make apt wait for the shared archives lock

The shared apt download cache is bind-mounted from outside the
chroot, so when an apt of ANOTHER build holds the archives lock from
its own pid namespace, a concurrent build's apt sees the holder as
"process 0", cannot wait for it, and fails the build immediately
(observed as an e2e samba essentials install racing a kernel build's
build-deps transaction on the stonking-amd64 cache).

Set DPkg::Lock::Timeout to ten minutes in the shared-cache apt conf:
apt then waits for the holder instead of erroring. pkh itself
already serializes builds per cache with a host-side flock; the
timeout covers the window where a killed run's orphaned apt is still
winding down.

src/deb/ephemeral.rs

@@ -320,7 +320,12 @@ pub(crate) fn write_apt_keep_conf(chroot_path: &Path) -> std::io::Result<()> {
     fs::create_dir_all(&conf_dir)?;
     fs::write(
         conf_dir.join(APT_KEEP_CONF),
-        "APT::Keep-Downloaded-Packages \"true\";\n",
+        // The lock timeout: the shared download cache is bind-mounted
+        // from outside the chroot, so an apt holding the archives lock
+        // in ANOTHER build's pid namespace shows up as "held by process
+        // 0" — a holder apt cannot see or wait for. Wait up to ten
+        // minutes instead of failing the build.
+        "APT::Keep-Downloaded-Packages \"true\";\nDPkg::Lock::Timeout \"600\";\n",
     )
 }
 
@@ -1593,7 +1598,9 @@ mod chroot_cleanup_tests {
     }
 
     /// The apt configuration keeping downloaded packages lands in the
-    /// chroot's apt.conf.d, so installs populate the shared cache.
+    /// chroot's apt.conf.d, so installs populate the shared cache; the
+    /// lock timeout makes apt wait for the shared archives lock instead
+    /// of failing on a holder in another pid namespace.
     #[test]
     fn apt_keep_conf_lands_in_the_chroot() {
         let dir = tempfile::tempdir().unwrap();
@@ -1601,7 +1608,10 @@ mod chroot_cleanup_tests {
         write_apt_keep_conf(&chroot).unwrap();
         let conf = chroot.join("etc/apt/apt.conf.d").join(APT_KEEP_CONF);
         let content = fs::read_to_string(&conf).unwrap();
-        assert_eq!(content, "APT::Keep-Downloaded-Packages \"true\";\n");
+        assert_eq!(
+            content,
+            "APT::Keep-Downloaded-Packages \"true\";\nDPkg::Lock::Timeout \"600\";\n"
+        );
 
         // Idempotent: a second write does not fail or duplicate.
         write_apt_keep_conf(&chroot).unwrap();