Target an even older version of Mac OS X

We can, so let's not ask if we should!

More realistically, we can right now so it's nice to mark that we do, but this is subject to be re-evaluated if any later changes conflicts with this.
Note also that this only affects the binaries we distribute with our releases!
It may very well be possible to compile RGBDS on even older versions of Mac OS X if you have the appropriate compiler setup;
we're just not testing it nor offering such binaries ourselves, but third-party packagers are welcome to do so.
(Looking at you, TigerBrew! ;)

.github/workflows/create-release-artifacts.yml

@@ -57,12 +57,14 @@ jobs:
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          ./.github/scripts/install_deps.sh macos
+          ./.github/scripts/install_deps.sh macos lld
       - name: Build binaries
         run: |
           cmake -S . -B build --preset macos-static -DFETCHCONTENT_BASE_DIR="${{ env.DEPS_ROOT_DIR }}"
           cmake --build build
           strip rgb{asm,link,fix,gfx}
+        env:
+          LDFLAGS: -fuse-ld=lld # cmake/macos-static.cmake comments explain why we use lld.
       - name: Package binaries
         run: |
           zip --junk-paths rgbds-macos.zip rgb{asm,link,fix,gfx} man/* .github/scripts/install.sh

.github/workflows/testing.yml

@@ -113,7 +113,7 @@ jobs:
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          .github/scripts/install_deps.sh macos
+          .github/scripts/install_deps.sh macos lld
       - name: Cache library deps
         uses: actions/cache@v5
         with:
@@ -124,6 +124,8 @@ jobs:
         run: |
           cmake -S . -B build --preset macos-static -DFETCHCONTENT_BASE_DIR="${{ env.DEPS_ROOT_DIR }}" -DTESTS_OS_NAME=macos
           cmake --build build
+        env:
+          LDFLAGS: -fuse-ld=lld # cmake/macos-static.cmake comments explain why we use lld.
       - name: Package binaries
         run: |
           mkdir bins