gcassert doesn't support Go 1.24: x/tools v0.17.0 can't parse new export format

[mw5h]

gcassert fails when run with Go 1.24 due to its dependency on golang.org/x/tools v0.17.0, which can't parse Go 1.24's new package export format:

2026/03/13 19:24:31 internal error: package "fmt" without types was imported from "github.com/cockroachdb/apd/v3"

This affects the current HEAD (389ef753) as well as older pinned versions like 7b67d223.

The fix should be to bump golang.org/x/tools to a version that supports Go 1.24 (e.g., v0.29.0 or later).

Discovered while updating CI for cockroachdb/apd#146.