default_prefix: set macOS default prefix on Linux if simulating it

[dawidd6]

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This should fix the doctor complaining on Linux in homebrew-core:

Warning: Your Homebrew's prefix is not /usr/local.
Some of Homebrew's bottles (binary packages) can only be used with the default
prefix (/usr/local).
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Twitter or any other official channels. You are responsible for resolving
any issues you experience while you are running this
unsupported configuration.

Above snippet taken from (somewhat companion) PR: Homebrew/homebrew-core#80082

The DEFAULT_PREFIX should not be set to the macOS one when setting HOMEBREW_FORCE_HOMEBREW_ON_LINUX.
Since we have stricter env var (HOMEBREW_SIMULATE_MACOS_ON_LINUX), then use it rather than the former.

Also, doctor was right complaining about this issue of course, because now every bottle built for Linux in homebrew-core has the prefix set to the macOS one and I gotta rebuild those formulae after this PR is merged.
Example of latest Linux bottle JSON with wrong prefix:

{
  "pkg-config": {
    "formula": {
      "name": "pkg-config",
      "pkg_version": "0.29.2_3",
      "path": "Library/Taps/homebrew/homebrew-core/Formula/pkg-config.rb",
      "tap_git_path": "Formula/pkg-config.rb",
      "tap_git_revision": "a7d01ae87028dd276476059772dd4a4db2aa6a63",
      "tap_git_remote": "https://github.com/Homebrew/homebrew-core",
      "desc": "Manage compile and link flags for libraries",
      "license": "GPL-2.0-or-later",
      "homepage": "https://freedesktop.org/wiki/Software/pkg-config/"
    },
    "bottle": {
      "root_url": "https://ghcr.io/v2/homebrew/core",
      "prefix": "/usr/local",
      "cellar": "any_skip_relocation",
      "rebuild": 0,
      "date": "2021-06-26",
      "tags": {
        "x86_64_linux": {
          "filename": "pkg-config-0.29.2_3.x86_64_linux.bottle.tar.gz",
          "local_filename": "pkg-config--0.29.2_3.x86_64_linux.bottle.tar.gz",
          "sha256": "3d9b8bf9b7b4bd08086be1104e3e18afb1c437dfaca03e6e7df8f2710b9c1c1a",
          "formulae_brew_sh_path": "formula",
          "tab": {
            "homebrew_version": "3.2.0-59-g196ec61",
            "changed_files": [
            ],
            "source_modified_time": 1490029698,
            "compiler": "gcc-5",
            "runtime_dependencies": [
            ],
            "arch": "x86_64",
            "built_on": {
              "os": "Linux",
              "os_version": "Ubuntu 16.04.7 LTS",
              "cpu_family": "broadwell",
              "glibc_version": "2.23",
              "oldest_cpu_family": "core2"
            }
          }
        }
      }
    }
  }
}

Note that we might need to swap those two env vars set in homebrew-core publishing workflow, but I'm not sure if the change in this PR will somehow negatively influence the publishing process. Talking about this line:
https://github.com/Homebrew/homebrew-core/blob/e80285c224946e333261436a9baa18a6faf26103/.github/workflows/publish-commit-bottles.yml#L14

[BrewTestBot]

Review period skipped due to critical label.

[dawidd6]

If set, running Homebrew on Linux will use URLs for macOS. This is useful when merging pull requests for macOS while on Linux.

From the manpage about HOMEBREW_FORCE_HOMEBREW_ON_LINUX. Nothing about paths.

[iMichka]

I'm ok with the change. I'll let your merge it when you have time to do the follow up tests in homebrew-core

[MikeMcQuaid]

Makes sense, thanks @dawidd6!