Add user_name environment variable

[kipkoan]

Add user_name environment variable (username of VCS user running command) for custom workflow commands to use.

[codecov]

Codecov Report

Merging #489 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   70.57%   70.57%   +<.01%     
==========================================
  Files          63       63              
  Lines        4258     4259       +1     
==========================================
+ Hits         3005     3006       +1     
  Misses       1028     1028              
  Partials      225      225

Impacted Files	Coverage Δ
server/events/runtime/run_step_runner.go	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #489 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   70.57%   70.57%   +<.01%     
==========================================
  Files          63       63              
  Lines        4258     4259       +1     
==========================================
+ Hits         3005     3006       +1     
  Misses       1028     1028              
  Partials      225      225

Impacted Files	Coverage Δ
server/events/runtime/run_step_runner.go	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce5e127...31b1910. Read the comment docs.

[lkysow]

Can you add this env var to the docs here: https://github.com/runatlantis/atlantis/blob/master/runatlantis.io/docs/atlantis-yaml-reference.md#custom-run-command

[kipkoan]

@lkysow - Do you prefer me to squash the commits?

[lkysow]

Yes please

[kipkoan]

@lkysow Done!

[lkysow]

Thanks!