cmd: make backend env vars show in help as the defaults for backend flags

Before this change

    RCLONE_DRIVE_CHUNK_SIZE=111M rclone help flags | grep drive-chunk-size

Would show the default value, not the setting of RCLONE_DRIVE_CHUNK_SIZE
as the non backend flags do.

This change makes it work as expected by setting the default of the
option to the environment variable.

Fixes #4659

cmd/cmd.go

@@ -513,7 +513,7 @@ func AddBackendFlags() {
 				if opt.IsPassword {
 					help += " (obscured)"
 				}
-				flag := pflag.CommandLine.VarPF(opt, name, opt.ShortOpt, help)
+				flag := flags.VarPF(pflag.CommandLine, opt, name, opt.ShortOpt, help)
 				if _, isBool := opt.Default.(bool); isBool {
 					flag.NoOptDefVal = "true"
 				}