pyodide sandbox option is insecure

Grist is spreadsheet software using Python as its formula language. Grist offers several methods for running those formulas in a sandbox, for cases where the user may be working with untrusted spreadsheets. One such method runs them in pyodide, but pyodide on node does not have a useful sandbox barrier. If a user of Grist sets `GRIST_SANDBOX_FLAVOR` to `pyodide` and opens a malicious document, that document could run arbitrary processes on the server hosting Grist. The problem has been addressed in Grist version 1.7.9 and up, by running pyodide under deno. As a workaround, a user can use the gvisor-based sandbox by setting `GRIST_SANDBOX_FLAVOR` to `gvisor`.