gnome.updateScript: More Python

This was prompted by the need to capture exit status of `find-latest-version.py` in the next commit.
I had to drop `errexit` (if I did not want to result to even worse hacks) but that concealed errors like the following,
when I accidentally used an incorrect equals operator in numeric comparison:

    line 24: ((: 1 = 1 : attempted assignment to non-variable (error token is "= 1 ")

Converting the plumbing in `gnome/update.nix` to Python also makes it slightly easier to read.

For now, `find-latest-version.py` is still invoked as a separate process (rather than being imported as a Python module),
as `update.nix` might be replaced by `genericUpdater` in the future.

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