Add nsd-pixi-shell.sh script for activating an env

Peterson, Peterrequested to merge
ewm14334_pixi_activate into main

Also modified the function to get the path to a manifest to allow for directories to be specified. This makes testing on other systems easier.

To test:

  1. nsd-pixi-wrap.sh prints an error an usage message
  2. nsd-pixi-wrap.sh --help prints the usage message
  3. nsd-pixi-wrap.sh /path/to/dir/with/pixi.toml runs pixi shell with that environment
  4. nsd-pixi-wrap.sh envname runs pixi shell with that environment that is installed in /usr/local/pixi
  5. nsd-pixi-wrap.sh garbageenv prints an error message

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