Detect unused dependency license metadata files

The "Check Go Dependencies" GitHub Actions workflow checks for dependencies with incompatible or unapproved license
types.

The dependency license metadata consumed by the "Licensed" tool is cached in the project repository, in a dedicated file
for each dependency.

The `check-cache` job of the workflow checks whether that cache is in sync with the project's current dependencies. It
does this by using the "Licensed" tool to update the cache and then a `git diff` command to check whether that resulted
in any changes (which would indicate it is out of sync).

Out of sync states could result from any of three distinct conditions:

- Missing metadata file
- Incorrect metadata file contents
- Superfluous metadata file

An incorrectly configured `git diff` command previously caused the last of these to be missed.

My first take at this system was simply using `git diff --exit-code` alone. That detects the last two, but misses the
first. I added the `git add --intent-to-add .` command to detect added files, but didn't realize that it caused the last
to be missed.

Superfluous files in the dependency license metadata cache won't actually interfere with its intended functionality, but
it is still important to avoid an accumulation of unused files.

The new commands will catch all three of the possible out of sync conditions by staging all changes that result from
the metadata cache update to the repository and then comparing those against the `HEAD` commit.

I considered an alternative approach which works just as well as the chosen one:

```
git add .
git diff --exit-code HEAD
```

However, I feel that the `--cached` flag makes the `git diff` command more self-explanatory.

Author: per1234

.github/workflows/check-go-dependencies-task.yml

@@ -63,8 +63,8 @@ jobs:
       - name: Check for outdated cache
         id: diff
         run: |
-          git add --intent-to-add .
-          if ! git diff --color --exit-code; then
+          git add .
+          if ! git diff --cached --color --exit-code; then
             echo
             echo "::error::Dependency license metadata out of sync. See: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/check-go-dependencies-task.md#metadata-cache"
             exit 1