Fix unit-test Prow failure by locking GOTOOLCHAIN to go1.24.0

This prevents Go from auto-downloading Go 1.25.0 minimal toolchain in Prow,
which contains a packaging bug where the 'covdata' tool is missing, causing
'-cover' test builds to fail with 'go: no such tool "covdata"'.

Locking it to go1.24.0 (which matches the go.mod baseline) ensures Go compiles
safely and avoids the bug, even on runners that only have an older Go version.

Author: yangspirit

Makefile

@@ -302,10 +302,10 @@ verify:
 	hack/verify-all.sh
 
 unit-test:
-	go test -v -mod=vendor -timeout 30s "./pkg/..." -cover
+	GOTOOLCHAIN=go1.24.0 go test -v -mod=vendor -timeout 30s "./pkg/..." -cover
 
 sanity-test:
-	cd test && go mod tidy && go test -mod=readonly -v -timeout 30s "./sanity/" -run TestSanity
+	cd test && go mod tidy && GOTOOLCHAIN=go1.24.0 go test -mod=readonly -v -timeout 30s "./sanity/" -run TestSanity
 
 build-e2e-test:
 	cd test && go build -o ../bin/e2e-test-ci ./e2e