Fix permissions for WAL data directory

[rfratto]

Fixes #1471

[rfratto]

(Do we have an easy way of testing this change? I'm pretty sure this will do it but instructions for manual testing should be something we document somewhere.)

cc @jdbaldry if you remember how to test these

[jdbaldry]

There's a test-packages target in the Makefile (https://github.com/grafana/agent/blob/main/Makefile#L477-L479). I'm not sure how much the underlying script would have degraded though so it may need some TLC. Let me know if you need someone to take a look at that and I'll put it on my list for tomorrow.

[rfratto]

Thanks Jack! I can look into it, no need for you to spend time on it.

Also, it's late for you, stop looking at Github :)

[tpaschalis]

Looks simple enough! ^^