Switch to Xcode12 and MacOS 10.15 for CI

[marciniwanicki]

• Updated CI image to use Xcode12
• Updated brew.sh script as the brew installation from URL does not work in the latest brew version. Likely @kwridan idea of using local<formula>.rb file works. It struggles with longer relative paths (i.e. brew install Scripts/Formulas/swiftlint.rb - I guess the path is treated as a tap/cask identifier), but works well if called with just filename (brew install swiftlint.rb).
• It looks the new macOS image comes with a preinstalled version of SwiftLint, tweaked the script to enforce using the version from the local formula.

Test Plan:

• CI

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@243cae5). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #67   +/-   ##
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  Coverage          ?   96.17%           
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  Files             ?       44           
  Lines             ?     1672           
  Branches          ?        0           
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  Hits              ?     1608           
  Misses            ?       64           
  Partials          ?        0           

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