Load PLUGIN_ENV_FILE before app is run

[joshdk]

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[joshdk]

As a preface, I have a Drone build use-case which provisions an external VM in one step, then runs commands on that host in subsequent steps. Part of the problem with this is that the address of the VM is dynamic and not know until after the machine is provisioned. And can not be hardcoded into .drone.yml.

To work around this, the VM provisioner writes PLUGIN_HOST=<random endpoint> to /drone/.env, and then I can then configure env_file: /drone/.env in my .drone.yml.

My PR takes direct inspiration from drone-plugins/drone-docker#89, including its security downsides. Im not sure if your collective opinion on this type of change has changed since then. Alternate solutions would include adding a host_file, a port_file, a ..._file, etc; which seemed sub-optimal.

In the event that there is a more idiomatic workaround, please let me know. And also, thank you for writing a fantastic plugin!