Use env vars first for credentials if no project is specified

[childish-sambino]

Fixes

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Contributing to Twilio

All third party contributors acknowledge that any contributions they provide will be made under the same open source license that the open source project is provided under.

Short description of what this PR does:

• Upstream change in core (PR #4) no longer defaults the project flag, so we must do it here

If you have questions, please file a support ticket, or create a Github Issue in this repository.

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   97.01%   97.02%   +0.01%     
==========================================
  Files          20       20              
  Lines         503      505       +2     
==========================================
+ Hits          488      490       +2     
  Misses         15       15

Impacted Files	Coverage Δ
src/commands/project/add.js	90.36% <100%> (+0.23%)	⬆️

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