Make HOMEBREW_NO_BOTTLE_SOURCE_FALLBACK default

[MikeMcQuaid]

• Remove HOMEBREW_NO_BOTTLE_SOURCE_FALLBACK and make the behaviour the default. We mostly already do this since we added the need for the --build-from-source override on macOS. This allows us to delete some more code.
• Still fail and require --build-from-source when reinstalling or upgrading if failing on a formula-specific reason e.g. the CLT is not/no longer installed, you're using a non-default prefix.

Fixes #10623

[BrewTestBot]

Review period will end on 2021-02-18 at 12:26:29 UTC.

[Bo98]

This doesn't cover the brew upgrade gcc case yet:

==> Upgrading 1 outdated package:
gcc 10.2.0_3 -> 10.2.0_4
==> Upgrading gcc 10.2.0_3 -> 10.2.0_4 
Warning: Building gcc from source:
  The bottle needs the Xcode CLT to be installed.
==> Downloading https://raw.githubusercontent.com/Homebrew/formula-patches/6a83f
Already downloaded: /Users/bo/Library/Caches/Homebrew/downloads/c0b9dca392d2e8496747001f634c04a4bc2f5c63672b963f3c4f217e70aaa258--bigsur_2.patch
==> Downloading https://ftp.gnu.org/gnu/gcc/gcc-10.2.0/gcc-10.2.0.tar.xz

[MikeMcQuaid]

This doesn't cover the brew upgrade gcc case yet:

@Bo98 Unset HOMEBREW_DEVELOPER.

[Bo98]

$ echo $HOMEBREW_DEVELOPER

$ brew upgrade gcc
==> Upgrading 1 outdated package:
gcc 10.2.0_3 -> 10.2.0_4
==> Upgrading gcc 10.2.0_3 -> 10.2.0_4 
Warning: Building gcc from source:
  The bottle needs the Xcode CLT to be installed.
==> Downloading https://raw.githubusercontent.com/Homebrew/formula-patches/6a83f
Already downloaded: /Users/bo/Library/Caches/Homebrew/downloads/c0b9dca392d2e8496747001f634c04a4bc2f5c63672b963f3c4f217e70aaa258--bigsur_2.patch
==> Downloading https://ftp.gnu.org/gnu/gcc/gcc-10.2.0/gcc-10.2.0.tar.xz

[MikeMcQuaid]

@Bo98 Have tweaked it a bit further. If that's still not working for you: can you try and look at check_install_sanity and figure out which part isn't acting as expected? Thanks! 🙇🏻

[Bo98]

That works:

$ brew upgrade gcc
==> Upgrading 1 outdated package:
gcc 10.2.0_3 -> 10.2.0_4
==> Upgrading gcc 10.2.0_3 -> 10.2.0_4 
Error: gcc: the bottle needs the Xcode CLT to be installed.
You can try to install from source with:
  brew install --build-from-source gcc
Please note building from source is unsupported. You will encounter build
failures with some formulae. If you experience any issues please create pull
requests instead of asking for help on Homebrew's GitHub, Twitter or any other
official channels.

[BrewTestBot]

Review period ended.