Disable macOS build for now due to compilation error

The current build fails to compile due to mismatching prototypes with
`interface=AGL` (the default), but works with `interface=FREEGLUT`.

For example, `interface=AGL` requires these changes to
`include/GL/glext.h` in order to compile:

```diff
-GLAPI void APIENTRY glTexParameterIivEXT (GLenum target, GLenum pname, const GLint *params);
-GLAPI void APIENTRY glTexParameterIuivEXT (GLenum target, GLenum pname, const GLuint *params);
+GLAPI void APIENTRY glTexParameterIivEXT (GLenum target, GLenum pname, GLint *params);
+GLAPI void APIENTRY glTexParameterIuivEXT (GLenum target, GLenum pname, GLuint *params);
```

that is, the last parameter is not `const` in the headers from Apple.

.github/workflows/ci.yml

@@ -32,8 +32,8 @@ jobs:
             coverage: true
           - perl-version: '5.30'
             os: windows-latest
-          - perl-version: '5.30'
-            os: macos-11
+          #- perl-version: '5.30'
+          #  os: macos-11
     steps:
       - uses: actions/checkout@v2
       - name: 'ci-dist: target-setup-perl'