BUG: Add default value for CF_TORCH_CUDA_ARCH_LIST_BACKUP in deactivate.sh

[carterbox]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

As reported here: conda-forge/cuda-nvcc-feedstock#92 and here: conda-forge/ctng-compiler-activation-feedstock#175, conda 26.1 will raise an error if an activate/deactivate script tries to evaluate an unbound environment variable.

In this MR, I add the default value syntax (:-) to the evaluation of CF_TORCH_CUDA_ARCH_LIST_BACKUP in the deactivate script such that if the activation script does not set this variable, then the deactivation script will not fail.

EDIT: This only seems to occur when gdb is also in the environment.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ The magma output has been superseded by libmagma-devel.
• ℹ️ The recipe is not parsable by parser conda-souschef (grayskull). This parser is not currently used by conda-forge, but may be in the future. We are collecting information to see which recipes are compatible with grayskull.
• ℹ️ The recipe is not parsable by parser conda-recipe-manager. The recipe can only be automatically migrated to the new v1 format if it is parseable by conda-recipe-manager.

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[h-vetinari]

Thanks for the PR. We'll queue this up with the next round of builds. :)