Fix osx-64 `base64` usage and add dependabot config

[jaimergp]

Comes from https://github.com/napari/packaging/issues/145#issuecomment-2149475205

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Nightlies have been failing in osx-64 for weeks due to:

base64: invalid argument --output
Usage:  base64 [-hDd] [-b num] [-i in_file] [-o out_file]
  -h, --help     display this message
  -Dd, --decode   decodes input
  -b, --break    break encoded string into num character lines
  -i, --input    input file (default: "-" for stdin)
  -o, --output   output file (default: "-" for stdout)

I assume something in the conda env is clobbering the system base64 with a different interface.