Fixed the non-functioning archive Grunt subcommand.

(Redmine issue: #3387)

Gruntfile.js

@@ -66,7 +66,7 @@ module.exports = function(grunt) {
             },
             // Archive previously built DEB packages from build directory.
             deb_archive: {
-                command: 'if [[ -n $(find <%= project_paths.deploy_dir %> -maxdepth 1 -name *.deb) ]]; then mv -f <%= project_paths.deploy_dir %>*.deb <%= project_paths.archive_dir %>; fi;'
+                command: 'pkgs=`find <%= project_paths.deploy_dir %> -maxdepth 1 -name *.deb` && if [ -n "$pkgs" ]; then mv -f <%= project_paths.deploy_dir %>*.deb <%= project_paths.archive_dir %>; fi;'
             },
             // Build DEB package.
             deb_build: {