add environment variable-based configuration

[droopy4096]

When using task in larger setups it would be more convenient to pass some of the configuration options via the environment variable so as not to update every line containing task invocation to add or remove options. Consider scenario of having several scripts with contents like:

#!/bin/sh

task target1
#....

task target2

to make them run in dry mode one has to alter all the shell scripts to include --dry option. However with environment variable configuration one can simply:

export TASK_DRY=yes

and run scripts with all task invocations picking up desired option automatically.

Of course --dry is only a sample, control of concurrency between workstation and CI node would be another good example. As well as increasing verbosity while debugging etc.

[jwater7]

I love this, exactly what I was looking for (specify the taskfile/entrypoint via an env var) - how do we get this in main so we can start using it?

[droopy4096]

someone will have to review and approve https://github.com/go-task/task/pull/1067