[starlark] Adjust to starlark API change.
[starlark] Adjust to starlark API change.

WriteBacktrace now requires strings.Builder, not bytes.Buffer.

TBR=2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org

Change-Id: I9513b2aed305058bf296bcd7352eb2910067be75
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1526249
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Ryan Tseng <2190fe9a39b4920ec7bd59c848ec5cccd163056f@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go