Platforms input list should be quoted

[sjberman]

A user of the action can supply platforms: "linux/arm64, linux/amd64" (note the space between the two platforms), but the double quotes are not persisted to the script call: platforms=${{ inputs.platforms }}. This will result in an error line 1: linux/amd64: No such file or directory.

If whitespace between platforms is to be supported, then quotes should be added to this input when calling the script.