Netlify Dev (Functions): Allow overriding env variables set in app.netlify.com with .env file

[RaeesBhatti]

- Do you want to request a feature or report a bug?
Bug

- What is the current behavior?
Env variables set in app.netlify.com override variables in .env files for functions when running with netlify dev.
- If the current behavior is a bug, please provide the steps to reproduce.

• Link your project to a Netlify site using netlify link
• Create an env variable in app.netlify.com for your site
• Create a .env file in your function directory with the same variable name (access that env variable in your function)
• Run netilfy dev and access your function, see the value for the env var

- What is the expected behavior?

• .env file in function directory overrides env vars set in app.netlify.com

- Local Environment Information

────────────────────┐
 Environment Info   │
────────────────────┘

  System:
    OS: macOS 10.15.3
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  Binaries:
    Node: 12.16.1 - ~/.n/bin/node
    Yarn: 1.17.3 - /usr/local/bin/yarn
    npm: 6.13.4 - ~/.n/bin/npm
  Browsers:
    Chrome: 80.0.3987.122
    Firefox: 72.0.2
    Safari: 13.0.5

netlify-cli/2.37.0 darwin-x64 node-v12.16.1

[RaeesBhatti]

This was initially reported here: #714 (comment)