Add tests on Go 1.22, and upgrade x/tools to make them work

[benjaminjkraft]

I guess our x/tools was too old? I'm unclear on how x/tools versioning works with language versioning. Anyway, now it all works. Fixes #312.

I have:

• [x] Written a clear PR title and description (above)
• [x] Signed the Khan Academy CLA
• [x] Added tests covering my changes, if applicable (n/a)
• [x] Included a link to the issue fixed, if applicable
• [x] Included documentation, for new features (n/a)
• [x] Added an entry to the changelog

[benjaminjkraft]

It's the thingy we use to load Go packages, to support package_bindings and other cases where we need to look at the caller's types