assets rake task: only try and detect brotli if COMPRESS_BROTLI is set

lib/tasks/assets.rake

@@ -115,6 +115,7 @@ def gzip(path)
   raise "gzip compression failed: exit code #{$?.exitstatus}" if $?.exitstatus != 0
 end
 
+if ENV['COMPRESS_BROTLI']&.to_i == 1
   # different brotli versions use different parameters
   ver_out, ver_err, ver_status = Open3.capture3('brotli --version')
   if !ver_status.success?
@@ -130,6 +131,7 @@ else
     # not sure what to do here, not expecting this
     raise "cannot determine brotli version"
   end
+end
 
 def brotli(path)
   if ENV['COMPRESS_BROTLI']&.to_i == 1