Optimization: avoid repeated interpolation in regexp

Benchmark.bm do |b|
  b.report("before") do
    100_000.times { /(\.#{MacOS.cat}\.bottle\.(\d+\.)?tar\.gz)$/ }
  end
  b.report("after ") do
    100_000.times { /(\.#{MacOS.cat}\.bottle\.(\d+\.)?tar\.gz)$/o }
  end
end
            user     system      total        real
before 35.400000   0.140000  35.540000 ( 35.619674)
after   0.020000   0.000000   0.020000 (  0.016662)