`get_running_shell` picks not-a-shell sometimes

Running `strace desk run hello something` leads to desk deciding my shell is `strace`, so it invokes `strace -ic something`. This actually runs something, but without the requested desk environment and a confusing strace warning and output.

Start by reproducing the issue with `strace desk run hello something` and tracing the `get_running_shell` entry point. Determine why `strace` is accepted as the shell, then verify that the command runs with the requested desk environment rather than invoking `strace` on `something`.

Written by the indexing model from the issue text.