fix: use format string in Fatalf to fix go vet error

[simonswine]

Problem

go vet (run as part of go test) now rejects non-constant format strings passed to Fatalf. This caused the CI workflow to fail on both amd64 and arm64 matrix jobs:

relay/remotequeue_flush_test.go:31:14: non-constant format string in call to (*testing.common).Fatalf
FAIL	github.com/pyroscope-io/pyroscope-lambda-extension/relay [build failed]

Fix

Replace string concatenation with a proper format string:

// before
j.t.Fatalf("should be still working... " + j.name)

// after
j.t.Fatalf("should be still working... %s", j.name)