Use Environment Variables when avalanchego-path or plugin-dir are not provided

[martineckardt]

Problem
On the command line it is easy to use the environment variables:

  avalanche-network-runner control start \
  --log-level debug \
  --endpoint="0.0.0.0:8080" \
  --number-of-nodes=5 \
  --avalanchego-path ${AVALANCHEGO_EXEC_PATH} \
  --plugin-dir ${AVALANCHEGO_PLUGIN_PATH} \
  --blockchain-specs '[{"vm_name": "subnetevm", "genesis": "./tests/precompile/genesis/evm-configuration-genesis.json"}]'

Unfortunately, that does not work with HTTP clients, such as Postman or ThunderClient. Therefore, we have to enter the paths in the HTTP Clients.

Possible solution
Is it possible to make execPath and pluginDir optional parameters in the control/start endpoint?
If they are not provided, then ANR should just use the contents of the environment variables AVALANCHEGO_EXEC_PATH and AVALANCHEGO_PLUGIN_PATH.