Remove loader process from heroku package (#43037)

### What does this PR do?
Remove the trace-loader process from the heroku package. 

### Motivation
Reduce package size.
The loader is currently unused on Heroku, and we'd rather reduce package size for this build.

### Describe how you validated your changes
CI.
Manually installing the deb package to make sure it won't break main's CI.

### Additional Notes
Somewhat re-apply #42965, but without breaking CI this time.

Co-authored-by: pierre.gimalac <pierre.gimalac@datadoghq.com>

Author: pgimalac

cmd/loader/main_noop.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+# noop loader for heroku
+# shouldn't be needed in practice, but it is used by the systemd service,
+# which is used when installing the deb manually
+
+shift # remove the path to the configuration file
+exec "$@"

omnibus/config/software/datadog-agent.rb

@@ -146,8 +146,15 @@
   end
 
   if linux_target?
-    command "dda inv -- -e loader.build --install-path=#{install_dir}", :env => env, :live_stream => Omnibus.logger.live_stream(:info)
-    copy "bin/trace-loader/trace-loader", "#{install_dir}/embedded/bin"
+    if heroku_target?
+      # shouldn't be needed in practice, but it is used by the systemd service,
+      # which is used when installing the deb manually
+      copy "cmd/loader/main_noop.sh", "#{install_dir}/embedded/bin/trace-loader"
+      command "chmod 0755 #{install_dir}/embedded/bin/trace-loader"
+    else
+      command "dda inv -- -e loader.build --install-path=#{install_dir}", :env => env, :live_stream => Omnibus.logger.live_stream(:info)
+      copy "bin/trace-loader/trace-loader", "#{install_dir}/embedded/bin"
+    end
   end
 
   if windows_target?