Merge #14115: lint: Make all linters work under the default macOS dev environment (build-osx.md)

341f7c7b0e macOS fix: Check for correct version of flake8 to avoid spurious warnings. The brew installed flake8 version is Python 2 based and does not work. (practicalswift)
908a559f33 macOS fix: Add excludes for checks added in the newer shellcheck version installed by brew (practicalswift)
ec4d57bbb3 macOS fix: Work around empty (sub)expression error when using BSD grep (practicalswift)
b57d7d92fe macOS fix: Avoid mapfile due to ancient version of bash shipped with macOS (practicalswift)

Pull request description:

  The linters are thoroughly tested under Ubuntu which is what we use in Travis. When reading #14041 I understood that some developers were experiencing problems when running the linters on their local machines.

  Assuming these local machines were running macOS I installed a fresh macOS VM, followed the instructions in `build-osx.md` and ran the linters.

  This PR contains the changes needed to make `lint-all.sh` run as expected.

  Ideally the linters would continuously run also under a Travis macOS environment to make sure we catch these kind of issues before merge.

Tree-SHA512: b39c9a970d14d27db1fb592539923c0bc676b5217f415d02fda3f17bf54d46faa172376e8a3ecab07ca68a3acba9aebe00b2b1b2161b2a36b85fbb672e7efb5c

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