Allow cache type mount

[hazelnutcloud]

I'd like to add a pip install command to the run field of the yaml file. To optimize this further, I tried adding a cache type mount with target set to /root/.cache/pip. When I tried building, this was the error output:

ⅹ There is a problem in your cog.yaml file. 
ⅹ build.run.0.mounts.0.type must be one of the following: "secret".
ⅹ 
ⅹ To see what options you can use, take a look at the docs:
ⅹ https://github.com/replicate/cog/blob/main/docs/yaml.md
ⅹ 
ⅹ You might also need to upgrade Cog, if this option was added in a
ⅹ later version of Cog.

I am running cog v0.9.4