github: Set -DCMAKE_INSTALL_MESSAGE=NEVER when testing libcxx

This matches what 28851edf164a337c334755ae33fd58f03cffd5a2 in
llvm-project did.

When setting up the built libcxx files for testing, the built
files are installed into a temporary test root, and the install of
them litters the build log with mostly uninteresting output.

Signed-off-by: Martin Storsjö <martin@martin.st>