fix: use env to generate config.Environ()

[caarlos0]

this will use the env's OnSet to record the variable names and values in order to generate config.Environ() results.

The side effect I can think of: if the config values are changed outside of the env after loading, they'll not be applied.

I see we have yaml tags too... if we load the settings from yaml, and then from env (which seems to be the case), it should be fine.

Also changed a bunch of tests to use t.Setenv instead of os.Setenv/os.Unsetenv

[codecov]

Codecov Report

Attention: 3822 lines in your changes are missing coverage. Please review.

Comparison is base (050a0d1) 50.52% compared to head (fab443e) 47.54%.
Report is 74 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #414      +/-   ##
==========================================
- Coverage   50.52%   47.54%   -2.99%     
==========================================
  Files         107      154      +47     
  Lines        8209    13021    +4812     
==========================================
+ Hits         4148     6191    +2043     
- Misses       3767     6342    +2575     
- Partials      294      488     +194     

Files	Coverage Δ
git/commit.go	42.85% <100.00%> (+17.85%)	⬆️
git/reference.go	40.00% <100.00%> (+7.85%)	⬆️
pkg/backend/backend.go	100.00% <100.00%> (ø)
pkg/backend/cache.go	68.75% <ø> (ø)
pkg/backend/context.go	85.71% <ø> (ø)
pkg/backend/settings.go	78.57% <ø> (ø)
pkg/config/config.go	80.00% <100.00%> (ø)
pkg/config/file.go	100.00% <ø> (ø)
pkg/config/ssh.go	100.00% <100.00%> (ø)
pkg/cron/cron.go	90.00% <100.00%> (ø)
... and 141 more

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.