Clean mullvadbrowser builds using browser-all-desktop instead of browser-all target

In #40076 (closed) we updated rbm-config/tb-build-06.torproject.org.rbm.local.conf to clean mullvadbrowser builds, however we used the browser-all target, which doesn't work since mullvadbrowser is not currently built for android. So we should use the browser-all-desktop target instead.

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