luci: Handle tag in the new github repo (#1410)

Since a6ec3cf0d3("perfetto: use Chromium mirror for LUCI (#1183)"), the
way tags are described in the checked out repo has changed.

Instead of v50.1, the tags show up as upstream/v50.1

Proof:
* For v49.0, the cipd package was tagged as v49.0
https://chrome-infra-packages.appspot.com/p/perfetto/trace_processor_shell/linux-arm64/+/wqR1jrSJAocyjh6cY-pJIE3QF6wfdTdkfkE2rk-rCYIC
* For v50.1, the cipd package is tagged as upstream/v50.1
https://chrome-infra-packages.appspot.com/p/perfetto/trace_processor_shell/linux-arm64/+/HXtZkPE1_oU74BLM0TFg3RNqojF0u4_A2dwKgxmUddwC

the tag used for cipd comes from `ctx.maybe_git_tag`, so we need to
fixed that.

Partially generated via `cd infra/luci && ./recipes.py test train`