The chromium build path created a TempDir via `tempdir()?.path().join(...)`,
which dropped the TempDir at the end of the statement and deleted the
directory before fetch_or_create_icon could write into it. Every
`deskify build --backend chromium` failed with:
Error: Failed to write RGBA PNG icon to /tmp/.tmpXXXXXX/icon.png
Caused by: No such file or directory (os error 2)
Bind the TempDir to a local like the tauri branch already does so it
lives until the icon has been written and copied to its final location.
