Add carbon to path during unix installs

[camdencheek]

Description of Changes

Links Carbon into /usr/local/bin so that it's available without a full path.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #28 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   76.03%   75.97%   -0.05%     
==========================================
  Files          61       61              
  Lines        3679     3679              
==========================================
- Hits         2797     2795       -2     
- Misses        661      663       +2     
  Partials      221      221              

Impacted Files	Coverage Δ
plugin/builtin/output/file.go	51.22% <0.00%> (-9.76%)	⬇️
plugin/builtin/input/file/read_to_end.go	72.88% <0.00%> (+3.39%)	⬆️

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