avoid mutating config while parsing -config.file

[owen-d]

Refactors our config loading to avoid accidentally mutating configs. Previously, in order to parse out the config.file flag, we'd have to call RegisterFlags + Parse on the Loki config struct and the global flagset. This plays poorly with slice flags. If they're defined as CLI flags but not in the YAML, they'll effectively get written twice into the config as multiple specifications of the flag appends.

Depends on cortexproject/cortex#2917
Fixes #2363

[slim-bean]

is it possible to write a test for the case that triggered this? change seems fine, config always seems harder than it needs to be.

[codecov-commenter]

Codecov Report

Merging #2392 into master will increase coverage by 0.02%.
The diff coverage is 45.83%.

@@            Coverage Diff             @@
##           master    #2392      +/-   ##
==========================================
+ Coverage   62.80%   62.83%   +0.02%     
==========================================
  Files         162      162              
  Lines       13966    13972       +6     
==========================================
+ Hits         8772     8779       +7     
+ Misses       4512     4506       -6     
- Partials      682      687       +5     

Impacted Files	Coverage Δ
pkg/cfg/cfg.go	50.00% <0.00%> (ø)
pkg/logcli/query/query.go	0.00% <0.00%> (ø)
pkg/cfg/files.go	33.33% <55.00%> (+26.01%)	⬆️
pkg/querier/queryrange/downstreamer.go	95.87% <0.00%> (-2.07%)	⬇️
pkg/promtail/targets/file/filetarget.go	67.85% <0.00%> (-1.79%)	⬇️
pkg/logql/evaluator.go	92.47% <0.00%> (-0.41%)	⬇️
pkg/promtail/targets/file/tailer.go	78.40% <0.00%> (+2.27%)	⬆️

[slim-bean]

LGTM