Replace "USERNAME" literals with a constant

pkg/shell/paths_windows.go

@@ -4,12 +4,14 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+
+	"github.com/elves/elvish/pkg/util"
 )
 
 // getSecureRunDir stats elvish-$USERNAME under the default temp dir, creating
 // it if it doesn't yet exist, and return the directory name.
 func getSecureRunDir() (string, error) {
-	username := os.Getenv("USERNAME")
+	username := os.Getenv(util.EnvUSERNAME)
 
 	runDir := filepath.Join(os.TempDir(), "elvish-"+username)
 	err := os.MkdirAll(runDir, 0700)

pkg/util/util.go

@@ -6,12 +6,13 @@ package util
 // Note that some of these env vars may be significant only in special
 // circumstances; such as when running unit tests.
 const (
+	EnvELVISH_TEST_TIME_SCALE = "ELVISH_TEST_TIME_SCALE"
 	EnvHOME                   = "HOME"
+	EnvLS_COLORS              = "LS_COLORS"
 	EnvPATH                   = "PATH"
 	EnvPATHEXT                = "PATHEXT"
 	EnvPWD                    = "PWD"
 	EnvSHLVL                  = "SHLVL"
-	EnvLS_COLORS              = "LS_COLORS"
+	EnvUSERNAME               = "USERNAME"
 	EnvXDG_RUNTIME_DIR        = "XDG_RUNTIME_DIR"
-	EnvELVISH_TEST_TIME_SCALE = "ELVISH_TEST_TIME_SCALE"
 )