Merge environment vars for "wrapped" tasks

[audunmo]

Say I have a taskfile like:

tasks:
  run:
    cmds:
      - env
    env:
      FOO: "bar"
  run:experimental
     cmds:
        - task: run
     env:
       EXPERIMENTAL: true

I'd want running task run:experimental to give the output:

...other env vars
FOO="bar"
EXPERIMENTAL=true
...other env vars

This is useful if you have a significant about of env vars, but want to override one or two for a particular case. Right now, the env from run:experimental is not inherited by the wrapped task