GHA: enable unity mode for cmake jobs + tidy-ups

Unity mode is not supported by CMake v3.7.2 used in linux-old, but
enable it anyway for consistency and to kick in automatically once

GHA: enable unity mode for cmake jobs + tidy-ups

Unity mode is not supported by CMake v3.7.2 used in linux-old, but
enable it anyway for consistency and to kick in automatically once
migrating to a newer old Linux in the future.

Also:
- replace `CMAKE_COMPILE_WARNING_AS_ERROR` with `CURL_WERROR`.
- delete default build option `PICKY_COMPILER=ON`.

Closes #13277

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CI: Add CI build on Debian stretch to test old support

This version still has ELTS support and contains some old versions of
key components like cmake to help prevent us from breaking th

CI: Add CI build on Debian stretch to test old support

This version still has ELTS support and contains some old versions of
key components like cmake to help prevent us from breaking that support.

Closes #13029

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