Use native go method to get current user rather than environment variable

[SagePtr]

Fixes #1640, #4894

Native "os/user" go package has good function to retrieve current user, rather than relying on USER environment variable.

If getting current user doesn't work on target OS (for example, not yet implemented for it) - it falls back to old implementation with environment variables.

Unit test also changed, old one won't work because USER environment variable spoofing doesn't work anymore (unless os/user is not implemented for OS under which tests are run), now it just compares CurrentUser() with whoami command output (skipped on Windows where whoami behaves different way).

For Windows OS with "domain\username" format, only second part (username) is returned to match previous behavior and trim unnecessary prefix.

[lafriks]

Blocked until 1.6.0-rc1 is released

[sapk]

Looks good just need a little change.

[codecov-io]

Codecov Report

Merging #4930 into master will increase coverage by <.01%.
The diff coverage is 27.27%.

@@            Coverage Diff            @@
##           master   #4930      +/-   ##
=========================================
+ Coverage    37.4%   37.4%   +<.01%     
=========================================
  Files         306     306              
  Lines       45378   45389      +11     
=========================================
+ Hits        16974   16980       +6     
- Misses      25949   25954       +5     
  Partials     2455    2455

Impacted Files	Coverage Δ
modules/user/user.go	23.52% <27.27%> (-76.48%)	⬇️
models/repo_list.go	63.37% <0%> (+1.16%)	⬆️
models/repo_indexer.go	52.11% <0%> (+2.54%)	⬆️

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