CI: move `CI` env variable to the root of the workflow. (#29499)
CI: move `CI` env variable to the root of the workflow. (#29499)

This way we match the Travis CI behavior, and it's inherited by all steps.
inway/bootstrap,twbs/bootstrap,peterblazejewicz/bootstrap,inway/bootstrap,coliff/bootstrap,m5o/bootstrap,m5o/bootstrap,tjkohli/bootstrap,peterblazejewicz/bootstrap,zalog/bootstrap,GerHobbelt/bootstrap,coliff/bootstrap,inway/bootstrap,tjkohli/bootstrap,zalog/bootstrap,GerHobbelt/bootstrap,GerHobbelt/bootstrap,peterblazejewicz/bootstrap,zalog/bootstrap,twbs/bootstrap