Make buildmessage fiber-aware

Port a simplified version of Meteor.EnvironmentVariable and
Meteor.bindEnvironment to fiber-helpers.js to deal with this.

Identify uses of fiberHelpers.inFiber and switch them to either
fiberHelpers.bindEnvironment (if the callback they are wrapping is
semantically "part of" the context that creates the callback) or
fiberHelpers.inBareFiber (otherwise).

Without this, concurrency was causing the wrong buildmessage message
sets and jobs to be active when builds yielded.