Reinterpret latest to mean "latest build"

[kevinAlbs]

Summary

Change latest to mean "latest build" rather than "latest release" for the downloaded server version.

Verified with this patch build in the C driver, which also includes changes from DRIVERS-3108.

Background & Motivation

Using latest as the server version currently downloads the latest release (including release candidates):

MONGODB_VERSION=latest ./.evergreen/run-orchestration.sh
mongosh --eval "db.version()"
# Prints: 8.0.5-rc2

Previously latest downloaded the latest build.

# Checkout commit before switching to mongodl.py:
git checkout 9142b7055ea5940e59ad41c4b069376f867031da
MONGODB_VERSION=latest ./.evergreen/run-orchestration.sh
mongosh --eval "db.version()"
# Prints: 8.1.0-alpha0-537-g60be604

I want drivers to test an unreleased server feature (SPM-2472). The switch from "latest build" to "latest release" seems unintended (slack). Drivers testing "latest build" seems more useful to catch test issues sooner.

Test failures

Testing this branch in the C driver resulted in two failures.

Load balancer behavior change

Latest build standalone tasks failed to run non-lb-connection-establishment.yml with error:

expected error to contain "Driver attempted to initialize in load balancing mode, but the server does not support this mode", but got: "Could not establish stream for node localhost:27017"

Filed SERVER-101078. A skip is added in DRIVERS-3108 as a short-term workaround.

macOS fail to start

A task on macOS 11 failed to start latest builds of mongod. Filed SERVER-101020.

[blink1073]

LGTM!