Fix incorrect exec transition for cargo_build_script

[UebelAndre]

Relying on an exec configuration through targets consumed in target leads to the potential for cfg = "exec" to be evaluated differently in the transitive target (in this case, cargo_build_script_runfiles.script) and the consumer (cargo_build_script.data_runfiles). This change moves the script directly back to cargo_build_script in cfg = "exec" and separately uses a cfg = "target" intermediate rule to implement runfiles (solving for bazelbuild/bazel#15486).

closes #3201 #3893

[slackito]

We've found a regression while testing this change internally. To reproduce, patch repro.patch into a clean working copy and run bazel build //test/unit/cargo_build_script_data_runfiles:reproduce_bin.

If I understand correctly, the main problem is that the runfiles library needs the RUNFILES_DIR env variable to point to the runfiles directory created by cargo_build_script. Apart from that, I believe we also need to copy the _repo_mapping file from the original runfiles dir to the new one. As far as I can tell, doing both of these things makes the example build correctly, but I don't know the right way to get the data to the right place. Could you please take a look?

[UebelAndre]

@slackito can you open a pull-request with the repro so I can see the failure?

[slackito]

@slackito can you open a pull-request with the repro so I can see the failure?

#4064