Weekly tests branches are not honored on scheduled workflow (valkey-io#3340)

`weekly.yml` calls `daily.yml` with `use_git_ref` set to each release
branch (for example 7.2). But the checkout logic in `daily.yml` only
used `inputs.use_git_ref` when `github.event_name` was
`workflow_dispatch` or `workflow_call`. otherwise it fell back to
`github.ref`.

For reusable workflows, GitHub keeps the caller workflow’s github
context. That means when `weekly.yml` is triggered by schedule, the
called `daily.yml` still sees `github.event_name == 'schedule'` and
`github.ref` for the caller branch (unstable). As a result, jobs labeled
as release-branch runs could still check out unstable.

Added a guard for Gtest Unit Tests. It will skip the job if gtest is not
available / supported.

Run with CI Issue:
https://github.com/valkey-io/valkey/actions/runs/22815380713

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Signed-off-by: Sarthak Aggarwal <sarthagg@amazon.com>

Author: sarthakaggarwal97