Limit parallel jobs on try/r+ runs
Limit parallel jobs on try/r+ runs

Rate limit of 60 applies org wide, let's not spam.
Thanks pietroalbini for the hint

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust