Make it easier for people to join the Build WG

[gibfahn]

I've been trying for a while to work out how we can help lower the barrier to entry for this WG, and nodejs/TSC#392 and the setup of the nodejs/automation team means now seems like a good time to suggest this.

Proposal

Split the test-access part of Build membership (and access to the secrets repo) into a subteam of build (in the same way we currently do release-access and infra-access). Then we can add people to the Build WG without worrying about who they are.

[mhdawson]

Discussed in latest (and I think in earlier meeting), no objections. Gibson to raise PR explaining approach.

[sam-github]

Closing as stale, but if anyone wants to take this up feel free to reopen.