forge: fix build
The build was failing due to mismatched libstdc++ between
stdenv.cc and cudatoolkit.cc. This can be fixed by bumping
the build-time cudatoolkit to 11, which is able to use the
same cc as stdenv.
(cherry picked from commit e263bdd82b3c91b0ad47412ac348a7dba3d0cb20)
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