DEV: Raise errors when cleaning the download cache, and fix for macOS (#8319)

POSIX's `head` specification states: "The application shall ensure that the number option-argument is a positive decimal integer"

Negative values are supported on GNU `head`, so this works in the discourse docker image. However, in some environments (e.g. macOS), the system `head` version fails with a negative `n` parameter.

This commit does two things:

Checks the status at each stage of the pipe, so it cannot fail silently
Flip the `ls` command to list in descending time order, and use `tail -n +501` instead of `head -n -500`.

The visible result is that macOS users no longer see head: illegal line count -- -500 printed throughout the test suite.

lib/file_store/base_store.rb

@@ -139,7 +139,13 @@ module FileStore
       FileUtils.mkdir_p(dir) unless Dir.exist?(dir)
       FileUtils.cp(file.path, path)
       # keep latest 500 files
-      `ls -tr #{CACHE_DIR} | head -n -#{CACHE_MAXIMUM_SIZE} | awk '$0="#{CACHE_DIR}"$0' | xargs rm -f`
+      processes = Open3.pipeline(
+        "ls -t #{CACHE_DIR}",
+        "tail -n +#{CACHE_MAXIMUM_SIZE + 1}",
+        "awk '$0=\"#{CACHE_DIR}\"$0'",
+        "xargs rm -f"
+      )
+      raise "Error clearing old cache" if !processes.all?(&:success?)
     end
 
     private