use shell to discover user-altered paths when escaping sandbox into host

Christian Hergert requested to merge wip/chergert/fix-rust-analyzer-on-host into main

The main thing here is that our programs spawned on the host are children of flatpak-session-helper and also have a reduced environment we can't control. If we want to regain the ability to use $PATH as altered by the user in something like .bashrc, we need to actually run the program within a shell.

Fixes #1384 (closed)

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