deb: point the build HOME at a chroot-internal scratch directory

The build context inherits pkh's environment, so the rules ran with
HOME set to the invoking user's host path - which does not exist
inside the chroot. Tools keying user config on HOME then fail
mid-build: gdb's doc build dies in texi2dvi because the TeX live
setup cannot use the nonexistent home.

Point HOME at /tmp/pkh-home, created through the build context (all
drivers: local, unshare chroot, ssh, schroot). sbuild uses a
non-writable path, Launchpad's buildd user a real one; a writable
scratch directory matches the latter.

src/deb/local.rs

@@ -148,6 +148,23 @@ pub async fn build(
         format!("parallel={} nocheck", num_cores),
     );
 
+    // The build context inherits pkh's environment: without an override,
+    // HOME is the invoking user's host path, which does not exist inside
+    // the chroot - tools keying user config on it (texi2dvi's TeX live
+    // setup, anything writing dotfiles) then fail mid-build. sbuild points
+    // HOME at a non-writable path, Launchpad's buildd user at a real one;
+    // a writable scratch directory matches the latter.
+    env.insert("HOME".to_string(), "/tmp/pkh-home".to_string());
+    let status = ctx
+        .command("mkdir")
+        .arg("-p")
+        .arg("/tmp/pkh-home")
+        .status()
+        .map_err(|e| format!("cannot create the build HOME: {e}"))?;
+    if !status.success() {
+        return Err("cannot create the build HOME (/tmp/pkh-home)".into());
+    }
+
     if cross {
         log::debug!("Setting up environment for local cross build...");
         cross::setup_environment(&mut env, arch, ctx.clone())?;