README is incorrect about default cache-key

[corneliusroemer]

The README currently says:

If cache-environment is set to true and cache-environment-key is not specified, setup-micromamba will use the default cache key (micromamba-environment). Similar behavior applies to cache-downloads and cache-downloads-key.

setup-micromamba/README.md

Line 195 in b09ef9b

If `cache-environment` is set to `true` and `cache-environment-key` is not specified, `setup-micromamba` will use the default cache key (`micromamba-environment`). Similar behavior applies to `cache-downloads` and `cache-downloads-key`.

This reads as if the default cache key was exactly and only micromamba-environment - not a good idea as that would mean all environments would have the same key.

So I added a more specific key.

However, I now see that the actual default key is much better, something like: micromamba-environment--linux-64-args-4f53cda-root-dcc80ee-bin-[object Promise]-file-20506a14f44f5955d29ad1066b5458ca7285ae3637add9c1971730bc89655bc5

So the docs are wrong and should be fixed. Also see #264 for the oddity of [object Promise] being in the cache key.