fix: don't use os.Environ to override env

[wolfogre]

If the env has been set in the yaml file, we shouldn't override it with os env.

[codecov]

Codecov Report

Merging #1629 (e92b6e0) into master (4989f44) will increase coverage by 0.66%.
The diff coverage is 75.93%.

@@            Coverage Diff             @@
##           master    #1629      +/-   ##
==========================================
+ Coverage   61.22%   61.88%   +0.66%     
==========================================
  Files          46       46              
  Lines        7141     7249     +108     
==========================================
+ Hits         4372     4486     +114     
+ Misses       2462     2459       -3     
+ Partials      307      304       -3     

Impacted Files	Coverage Δ
pkg/container/docker_cli.go	82.23% <ø> (ø)
pkg/container/docker_logger.go	52.08% <ø> (ø)
pkg/container/docker_pull.go	33.33% <ø> (ø)
pkg/container/docker_run.go	13.58% <ø> (ø)
pkg/container/docker_volume.go	0.00% <ø> (ø)
pkg/container/docker_images.go	27.02% <29.41%> (-4.13%)	⬇️
pkg/model/github_context.go	61.06% <32.00%> (-19.40%)	⬇️
pkg/container/docker_auth.go	51.35% <56.25%> (+3.73%)	⬆️
pkg/runner/runner.go	86.45% <66.66%> (-2.05%)	⬇️
pkg/runner/step.go	83.33% <75.00%> (+0.36%)	⬆️
... and 18 more

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[ChristopherHX]

Thank you, Good catch.

at a later point of time, we should eventually move this code block into the Exec function of the HostEnvironment struct, so env vars defined in environ are not inside rc.Env. Like already done for the docker executor