Build-env image overwriting

[namesty]

Closes #616

Only in the case where the user builds 2 wrappers without specifying a build manifest in any of them, would the image overwriting happen. However, if the manifest's declared language changed, we would need to reconstruct the build/env cache directory; else the directory should be reused (so that the uuid does not change between builds of the same project). Therefore, the implementation is:

1. Cache web3api.yaml to cache the language with which the cache build/env dir was created
2. Stop reconstructing the cache build/env dir each time the project builds
3. If language did not change or no previous build/env is found in the cache dir, then copy default env files, adding the uuid suffix to the image name.