x/exp,x/mobile/gl: build failure on darwin-arm64-11_0-toothrot due to missing OpenGL ES headers

[bcmills]

Note that the build failure here is on an x/mobile library, but the failure is observed while building the x/exp repo.

2020-12-15T15:35:30-b5a6e24/darwin-arm64-11_0-toothrot

# golang.org/x/mobile/gl
In file included from /tmp/buildlet/gopath/pkg/mod/golang.org/x/mobile@v0.0.0-20190719004257-d2bd2a29d028/gl/work.go:29:
./work.h:18:10: fatal error: 'OpenGLES/ES2/glext.h' file not found
#include <OpenGLES/ES2/glext.h>
         ^~~~~~~~~~~~~~~~~~~~~~
1 error generated.

CC @golang/release @hyangah

[dmitshur]

I missed this and made a duplicate issue #43237. This was fixed, see that issue for details.