Fix gh-actions on Mac Py 3.10.

Do this by exporting `_PYTHON_HOST_PLATFORM="macosx-11-x86_64"` on macos-latest.
Strangely, this works.
See buildout/buildout#609 (comment)

.github/workflows/run-buildout.yml

@@ -15,10 +15,6 @@ jobs:
         - ubuntu-latest
         - windows-latest
         - macos-latest
-        exclude:
-        # MacOS on gh-actions cannot install MarkupSafe on Py 3.10, see https://github.com/plone/buildout.coredev/issues/799
-        - os: macos-latest
-          python-version: "3.10"
     runs-on: ${{ matrix.os }}
 
     steps:
@@ -40,7 +36,13 @@ jobs:
         key: eggs-${{ matrix.python-version }}-${{ runner.os }}-${{ hashFiles('versions*.cfg') }}
         restore-keys: |
           eggs-${{ matrix.python-version }}-${{ runner.os }}-
-          eggs-${{ matrix.python-version }}-
+    - name: Run buildout if Mac
+      if: matrix.os == 'macos-latest'
+      run: |
+        # MacOS on gh-actions cannot install MarkupSafe on Py 3.10, see https://github.com/plone/buildout.coredev/issues/799
+        export _PYTHON_HOST_PLATFORM="macosx-11-x86_64"
+        buildout buildout:git-clone-depth=1
     - name: Run buildout
+      if: matrix.os != 'macos-latest'
       run: |
         buildout buildout:git-clone-depth=1