refactor: setup context

[crazy-max]

Set up context and move goreleaser dedicated logic in its own namespace to start working on artifacts and metadata outputs.

Also properly set env vars for exec.exec in main.

[codecov]

Codecov Report

Merging #325 (7baddcd) into master (a80c8fd) will decrease coverage by 0.36%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   79.61%   79.24%   -0.37%     
==========================================
  Files           4        4              
  Lines         103      106       +3     
  Branches       18       18              
==========================================
+ Hits           82       84       +2     
- Misses         13       14       +1     
  Partials        8        8              

Impacted Files	Coverage Δ
src/context.ts	83.33% <83.33%> (ø)
src/github.ts	86.66% <83.33%> (ø)
src/goreleaser.ts	76.08% <85.71%> (ø)

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