Allow user-specified build strings

[MartinJepsen]

I'm using the Python build backend and there is no (documented) way of supplying your own build string.

I'm not sure of the possibilities here, but adding jinja support (if not present) would be really great. This is possible with rattler-build, which I was using before.

What I'm trying to do

In my company, we use the conda-build inspired build string / build number template of <commit hash>_<no. of commits since latest tag>. I was able to do this in rattler-build by writing a small preprocessing script that sets environment variables for the commit hash and number of commits since latest tag (as output by git describe --tags --abbrev=8), and then using jinja templates in my recipe.yaml for using those environment variables.

Currently with the Python pixi build backend, it seems that the build string is invariant with respect to changes in the source code (so it can't be the package hash, right?).

[MartinJepsen]

@baszalmstra @ruben-arts I hope you guys are the right ones to ask. I would like to give this implementation a try, but I'm not really sure how to go about it.

I would imagine something simple like

[package]
name = ...
version = ...
build-string = "${{ env.get("BUILD_STR") }}"

Where BUILD_STR is an environment variable set prior to rendering the recipe.

[tdejager]

Yeah you are correct, we are not able to set that just yet!