tools.vpm: debug-log to $VMODULES/cache/vpm.log if not running in debug mode (#21192)

cmd/tools/vpm/settings.v

@@ -19,17 +19,27 @@ mut:
 	fail_on_prompt bool
 	// git is used by default. URL installations can specify `--hg`. For already installed modules
 	// and VPM modules that specify a different VCS in their `v.mod`, the VCS is validated separately.
-	vcs VCS
+	vcs    VCS
+	logger &log.Logger
 }
 
 fn init_settings() VpmSettings {
 	args := os.args[1..]
 	opts := cmdline.only_options(args)
 	cmds := cmdline.only_non_options(args)
-	if os.getenv('VPM_DEBUG') != '' {
-		log.set_level(.debug)
-	}
+
+	vmodules_path := os.vmodules_dir()
 	no_inc_env := os.getenv('VPM_NO_INCREMENT')
+	dbg_env := os.getenv('VPM_DEBUG')
+
+	mut logger := &log.Log{}
+	if dbg_env != '0' {
+		logger.set_level(.debug)
+		if dbg_env == '' {
+			logger.set_output_path(os.join_path(vmodules_path, 'cache', 'vpm.log'))
+		}
+	}
+
 	return VpmSettings{
 		is_help: '-h' in opts || '--help' in opts || 'help' in cmds
 		is_once: '--once' in opts
@@ -37,9 +47,10 @@ fn init_settings() VpmSettings {
 		is_force: '-f' in opts || '--force' in opts
 		server_urls: cmdline.options(args, '--server-urls')
 		vcs: if '--hg' in opts { .hg } else { .git }
-		vmodules_path: os.vmodules_dir()
+		vmodules_path: vmodules_path
 		tmp_path: os.join_path(os.vtmp_dir(), 'vpm_modules')
 		no_dl_count_increment: os.getenv('CI') != '' || (no_inc_env != '' && no_inc_env != '0')
 		fail_on_prompt: os.getenv('VPM_FAIL_ON_PROMPT') != ''
+		logger: logger
 	}
 }