Prepare for Loopvar scope change

[muncus]

go 1.21 includes an experiment to evaluate a proposed change in variable scoping of for loops: https://github.com/golang/go/wiki/LoopvarExperiment

before 1.22 is released, we should run all tests in this repo with this experiment enabled, to get a sense for whether it will work, and what side effects it may have.

Followup bugs can be filed for any issues identified.

[muncus]

Listing as p1 for now until we can determine if this blocks adoption of 1.22. if we're ok to move forward, we can downgrade to p2.

[muncus]

this looks to be a noop for our current tests, as determined by testing in #3756