cmd/go: go bug uses predictable temporary filenames [1.26 backport]

[gopherbot]

@neild requested issue #78584 to be considered for backport to the next 1.26 minor release.

@gopherbot please open backport issues for this security issue.

[gopherbot]

Change https://go.dev/cl/763883 mentions this issue: [release-branch.go1.26] cmd/go: use MkdirTemp to create temp directory for "go bug"