Improve recipe debugging

[wolfv]

Some ideas for a better debug experience:

When we enter a rattler-build working directory we could add the following methods to rattler-build:

• rattler-build diff: should diff the (first) source against the current changes in the work dir – for this we can invoke git diff
• rattler-build install --env host foobar: install the foobar dependency into the host environment (same for build)
• rattler-build execute-build: re-execute conda-build.sh (although this might not really be necessary)

[wolfv]

I have started to work on a prototype using diffy here: https://github.com/wolfv/rattler-build/tree/create-patch-functions