New version of `golang.org/x/exp` breaks tests.

[dezlitz]

1. Run make test, OK.
2. go get go get golang.org/x/exp@latest
3. Run make test, fails with error, internal/variable/registryset.go:62:3: type func(a RegisteredVariable, b RegisteredVariable) bool of func(a, b RegisteredVariable) bool {…} does not match inferred type func(a RegisteredVariable, b RegisteredVariable) int for func(a E, b E) int.