loki.source.journald: sync positions directory permissions

[tpaschalis]

PR Description

This PR fixes the permissions which are used to create the positions directory for loki.source.journal to match all the other loki.source.* components. It also moves the directory creation to before the definition of the file.

Eg. here's how it works for loki.source.docker

agent/component/loki/source/docker/docker.go

Lines 77 to 85 in 4b8807a

	err := os.MkdirAll(o.DataPath, 0750)
	if err != nil && !os.IsExist(err) {
	return nil, err
	}
	positionsFile, err := positions.New(o.Logger, positions.Config{
	SyncPeriod: 10 * time.Second,
	PositionsFile: filepath.Join(o.DataPath, "positions.yml"),
	IgnoreInvalidYaml: false,
	ReadOnly: false,

Which issue(s) this PR fixes

Fixes #3972

Notes to the Reviewer

Nothing in particular. Maybe this should also be part of the shared abstraction for logging components.

PR Checklist

• CHANGELOG updated
• Documentation added (N/A)
• Tests updated (N/A)

[thampiotr]

LGTM! Agreed that it should ideally be part of some common library or part of a wrapper around the positions structs, but it's higher priority to fix the issue first.