Update .gitignore for ignoring goenv .go-version file

[siansiansu]

Hi reviewers:

I updated the .gitignore to ignore the .go-version file in this PR.
If merged, it's easy to manage different go version locally for the goenv user.
Please help to review it. Thanks!

Reference
https://github.com/syndbg/goenv/blob/master/COMMANDS.md#goenv-local

[siansiansu]

I have signed the CLA, please help to re-run it if approved.

[gmlewis]

Thank you, @siansiansu !
Let's please make the comment a bit more useful.

[codecov]

Codecov Report

Merging #2242 (87a1844) into master (4d7a0b5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2242   +/-   ##
=======================================
  Coverage   97.80%   97.80%           
=======================================
  Files         113      113           
  Lines       10205    10205           
=======================================
  Hits         9981     9981           
  Misses        156      156           
  Partials       68       68           

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[gmlewis]

Thank you, @siansiansu !
LGTM.
Merging.