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ffc0afb0 - [Mosaic GPU] Attempt no. 2 at fixing the Mac OS build.

[Mosaic GPU] Attempt no. 2 at fixing the Mac OS build. Now following [the recommendations from gnu.org](https://www.gnu.org/software/gnulib/manual/html_node/environ.html) for how to deal with `environ` not being defined. PiperOrigin-RevId: 796835110