fix(desktop-update): discard failed profile allocation output

scripts/desktop-update/posix.sh

@@ -246,6 +246,7 @@ start_ui() {
   { [ -f "$html" ] && [ -n "$py" ] && [ -n "$browser" ]; } || { log "shim: no renderer; skipping UI"; return; }
 
   UI_PROFILE_DIR="$(mktemp -d "${TMPDIR:-/tmp}/hermes-update-ui-XXXXXXXX")" || {
+    UI_PROFILE_DIR=""
     log "shim: could not allocate a browser profile; skipping UI"
     return
   }

tests/test_desktop_update_shim_profile_cleanup.py

@@ -22,7 +22,7 @@ def test_shim_removes_only_its_owned_profile(tmp_path, outcome):
     browser.chmod(0o755)
     if outcome == "allocation-failed":
         allocator = bin_dir / "mktemp"
-        allocator.write_text("#!/bin/sh\nexit 1\n", encoding="utf-8")
+        allocator.write_text('#!/bin/sh\nprintf "%s\\n" "$HOME"\nexit 1\n', encoding="utf-8")
         allocator.chmod(0o755)
     config = tmp_path / ".config"
     config.mkdir()