ci: partial go cache fallback in backport workflow (#10658)

actions/setup-go's built-in cache uses a single key derived from
`hash(go.sum)` with no `restore-keys` fallback, so any divergence
between a release branch's `go.sum` and `main` is a full miss.
`release-1.25` hits this almost every time and re-downloads the full
module set, adding roughly two minutes per backport.

Disable the built-in cache and use `actions/cache` with a per-branch key
plus a two-level `restore-keys` ladder, so a diverged `go.sum` reuses
the most recent cache for the same branch (or any branch) and only the
delta is downloaded.

Closes #10657

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>