DEV: Cap number of `thread-loader` workers in `assets:precompile:build` (#28830)

We were running into errors running `ember build` on machines with high
CPU counts. It was then noted that `thread-loader`, which embroider uses, defaults to spinning
up x workers where x is number of physical CPU cores - 1. That is
probably too much so we set out to find out an optimial count to set for
the `JOBS` env which embroider will use to set the number of
`thread-loader` workers.

I first built an image using the following Dockerfile.

```
FROM discourse/base:release

RUN cd /var/www/discourse && sudo -EH -u discourse bundle exec rake plugin:install_all_official
RUN cd /var/www/discourse && sudo -EH -u discourse bundle exec rake assets:precompile:prereqs
```

I then ran the following command on my M3 Max Macbook Pro that has 14
phyisal CPU cores.

```
for j in 1 2 4 8 14; do echo "JOBS=$j"; time docker run --rm -it -e JOBS=$j test:latest /bin/bash -c "su discourse -c 'cd /var/www/discourse && bundle exec rake assets:precompile:build'"; done
```

These are the results I got:

```
JOBS=1 0.04s user 0.03s system 0% cpu 1:01.92 total
JOBS=2 0.04s user 0.02s system 0% cpu 42.605 total
JOBS=4 0.04s user 0.02s system 0% cpu 37.012 total
JOBS=8 0.04s user 0.02s system 0% cpu 35.199 total
JOBs=14 0.04s user 0.02s system 0% cpu 37.941 total
```

We think JOBS=2 is a good default when the `JOBS` env has not been set.
Anything above just consumes more resources for little benefit.

lib/tasks/assets.rake

@@ -12,18 +12,26 @@ task "assets:precompile:build" do
 
     raise "Unknown ember version '#{ember_version}'" if !%w[5].include?(ember_version)
 
+    # If `JOBS` env is not set, `thread-loader` defaults to the number of CPUs - 1 on the machine but we want to cap it
+    # at 4 because benchmarking has shown that anything beyond 4 does not improve build times or the increase is marginal.
+    # Therefore, we cap it so that we don't spawn more processes than necessary.
+    jobs_env_count = (4 if !ENV["JOBS"].present? && Etc.nprocessors > 4)
+
     compile_command = "CI=1 pnpm --dir=app/assets/javascripts/discourse ember build"
 
     heap_size_limit = check_node_heap_size_limit
 
     if heap_size_limit < 2048
       STDERR.puts "Node.js heap_size_limit (#{heap_size_limit}) is less than 2048MB. Setting --max-old-space-size=2048 and CHEAP_SOURCE_MAPS=1"
+      jobs_env_count = 0
+
       compile_command =
-        "JOBS=0 CI=1 NODE_OPTIONS='--max-old-space-size=2048' CHEAP_SOURCE_MAPS=1 #{compile_command}"
+        "CI=1 NODE_OPTIONS='--max-old-space-size=2048' CHEAP_SOURCE_MAPS=1 #{compile_command}"
     end
 
     ember_env = ENV["EMBER_ENV"] || "production"
     compile_command = "#{compile_command} -prod" if ember_env == "production"
+    compile_command = "JOBS=#{jobs_env_count} #{compile_command}" if jobs_env_count
 
     only_ember_precompile_build_remaining = (ARGV.last == "assets:precompile:build")
     only_assets_precompile_remaining = (ARGV.last == "assets:precompile")
@@ -230,11 +238,13 @@ def concurrent?
   if ENV["SPROCKETS_CONCURRENT"] == "1"
     concurrent_compressors = []
     executor = Concurrent::FixedThreadPool.new(Concurrent.processor_count)
+
     yield(
       Proc.new do |&block|
         concurrent_compressors << Concurrent::Future.execute(executor: executor) { block.call }
       end
     )
+
     concurrent_compressors.each(&:wait!)
   else
     yield(Proc.new { |&block| block.call })