Update default cuda architecture.

- Cuda 11 dropped support for -arch=sm_30. The default was therefore updated to sm_35.
- Furthermore, it was impossible to select virtual architectures such as compute_70,
  since sm_ is always prepended to the CMake variable.
  Now, one specifies
    -DCUDA_ARCH=sm_35
  instead of
    -DCUDA_ARCH=35
4 jobs for master in 9 minutes and 24 seconds (queued for 1 second)
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clang-format Build
Processing triggers for dbus (1.12.16-2ubuntu2.1) ...
$ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add
--2021-02-01 18:29:58-- https://apt.llvm.org/llvm-snapshot.gpg.key
Resolving apt.llvm.org (apt.llvm.org)... Warning: apt-key output should not be parsed (stdout is not a terminal)
failed: Temporary failure in name resolution.
wget: unable to resolve host address 'apt.llvm.org'
gpg: no valid OpenPGP data found.
Cleaning up file based variables
ERROR: Job failed: exit code 1