ci: cache the license-eye build in the Check License Headers job

[Amer-Mukhtar]

What changes were proposed in this PR?

The Check License Headers job uses the apache/skywalking-eyes action, which sets
up Go and builds the license-eye tool from source on every run. Building the tool
takes about as long as the scan it exists to do.

This PR keeps the action but caches the binary it installs. On an empty cache
nothing changes: the action builds the tool, installs it, and scans, and the
binary is saved at the end. On later runs the binary is restored, the action is
skipped, and the job goes straight to the scan.

The cache key is the hash of the workflow file. The action version is pinned in
that same file, so bumping the pin changes the key and the tool is rebuilt. If
the key held a copy of the pinned commit instead, a Renovate bump would leave it
stale and later runs would keep using the old binary.

The action is still referenced with a uses line rather than replaced by a direct
go install, because Renovate only tracks dependencies written that way.

Cold runs cost the same as before, so the saving shows up only on warm runs.

Any related issues, documentation, discussions?

Closes #7136

How was this PR tested?

Workflow files have no test framework, so the check is CI on this pull request.
The first run is a cache miss and should behave exactly as it does today.
Re-running the job takes the warm path, and its log should show the scan with no
Go setup or build step before it. Both runs need to pass.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude Code

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