fix: adapt walker_test to NewTargetHashCache 5-arg signature (#4)

## Summary

Main is broken: `//pkg:pkg_test` fails to compile because
`pkg/walker_test.go` still calls the 4-arg `NewTargetHashCache`, while
the [`--rule-class-fingerprint`
feature](#2) added a
5th parameter (\`ruleClassFingerprints []RuleClassFingerprintDigests\`).

Failing CI:
https://github.com/spotify/target-determinator/actions/runs/24855629042/job/72767594737

\`\`\`
pkg/walker_test.go:33:63: not enough arguments in call to
NewTargetHashCache
    have (nil, *Normalizer, string, bool)
want (map[label.Label]map[Configuration]*analysis.ConfiguredTarget,
*Normalizer, string, bool, []RuleClassFingerprintDigests)
\`\`\`

\`walker_test.go\` is a fork-local test file that I missed when updating
call sites during #2. Pass \`nil\` for the new parameter — the test
doesn't exercise fingerprints.

## Test plan

- [x] \`bazel test //pkg:pkg_test\` passes locally.
- [x] CI presubmit green.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: honnix

pkg/walker_test.go

@@ -30,7 +30,7 @@ func TestDiffSingleLabel_NoDifferenceWhenBothFromCache(t *testing.T) {
 	hashKey := lbl.String() + "\x00" + config.String()
 
 	makeFromCache := func() *QueryResults {
-		thc := NewTargetHashCache(nil, &Normalizer{}, bazelRelease, false)
+		thc := NewTargetHashCache(nil, &Normalizer{}, bazelRelease, false, nil)
 		if err := thc.RestoreHashes(map[string][]byte{hashKey: fakeHash}); err != nil {
 			t.Fatalf("RestoreHashes: %v", err)
 		}