feat(run): Set INIT_CWD in environment

[DimitarNestorov]

Summary
INIT_CWD environment variable holds the full path you were in when you ran yarn run

Motivation: https://twitter.com/housecor/status/983773854530260992

Test plan

1. Create a script called testscript.js
2. In the file type console.log(process.env.INIT_CWD);
3. In package.json add a script with a key testscript and value node testscript.js
4. Build yarn
5. Call ../bin/yarn testscript from the command line in a subfolder
6. The script should log the folder you're running the script from

Video: https://www.youtube.com/watch?v=XY_w8GVtuCY

Test plan

Added a new test.

[BYK]

Great and concise patch, thanks a lot!

[DimitarNestorov]

Thank you @BYK!

Just wanted to ask did ci/circleci: test-linux-node4 fail because of outside forces or RNG? If yes is there a way to rerun it in case I believe it did fail randomly?

I can't imagine writing a test fixes another test 😄

[BYK]

@DimitarNestorov I don't see the failure :S

[DimitarNestorov]

@BYK It's on the first commit of the pull request.

[BYK]

@DimitarNestorov don't worry about that one, probably was a fluke :)