Add gha-runner repository under automation

[Kobzol]

Repo: https://github.com/rust-lang/gha-runner

Extracted from GH:

org = "rust-lang"
name = "gha-runner"
description = "Custom fork of the GitHub Actions runner"
bots = []

[access.teams]
infra = "write"
security = "pull"

[access.individuals]
pietroalbini = "admin"
shepmaster = "write"
Kobzol = "write"
jdno = "admin"
rylev = "admin"
badboy = "admin"
Mark-Simulacrum = "admin"
kennytm = "write"
rust-lang-owner = "admin"

[Kobzol]

Changed to infra-admins.