tidb-server should not persist ScopeNone sysvars during bootstrap

[morgo]

Enhancement

Discovered in #28555

It looks like during bootstrap the value of ScopeNone sysvars is persisted to mysql.global_variables, but this is never consulted when the sysvar is read:

tidb/sessionctx/variable/varsutil.go

Lines 172 to 182 in d458059

	// GetSessionOrGlobalSystemVar gets a system variable.
	// If it is a session only variable, use the default value defined in code.
	// Returns error if there is no such variable.
	func GetSessionOrGlobalSystemVar(s *SessionVars, name string) (string, error) {
	sv := GetSysVar(name)
	if sv == nil {
	return "", ErrUnknownSystemVar.GenWithStackByArgs(name)
	}
	if sv.HasNoneScope() {
	return sv.Value, nil
	}

If the intention is to persist, the var should be instead be changed to global Scope + set as read only instead.

Most of the ScopeNone sysvars are instance scoped (not global) and overwritten during server startup. So this is not causing any current bugs, it just looks confusing when inspecting the table.