Updater: cross-platform compatibility, robustness enhancements

[mistydemeo]

This makes a few changes to the updater with the goal of ensuring it runs across all platforms and that it requires fewer external tools. Since these tools aren't preinstalled on Windows, it should ensure that running on Windows just works the same as on other platforms. In particular:

• The external curl call has been replaced by a call to reqwest, which was already used in the crate for other things. This should also ensure it works on minimal Linux docker images, some of which come with wget but no curl preinstalled.
• The calls to Unix commands to move and rename files have been replaced with calls to the Rust stdlib.
• Executable file permissions are handled via Rust stdlib calls, and are now only run on Unix; these permissions don't exist on Windows.
• The call to tar -xzf has been replaced with Rust calls to decompress the tarball using the flate2 crate and then extract using the tar crate, ensuring that tarball extraction will be possible on Windows.