Fix RERUN_C_BUILD_ARTIFACT path value if CARGO_BUILD_TARGET env variable is set

[traversaro]

What

When building the conda-forge package for the C++ bindings for rerun-sdk, I encountered a failure of the following type (see conda-forge/staged-recipes#25648 (comment)):

[307/532] Linking CXX shared library rerun_cpp/librerun_sdk.so
FAILED: rerun_cpp/librerun_sdk.so 
: && $BUILD_PREFIX/bin/x86_64-conda-linux-gnu-c++ -fPIC -fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/librerun-sdk-0.14.1 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix -O3 -DNDEBUG  -Wl,-O2 -Wl,--sort-common -Wl,--as-needed -Wl,-z,relro -Wl,-z,now -Wl,--disable-new-dtags -Wl,--gc-sections -Wl,--allow-shlib-undefined -Wl,-rpath,$PREFIX/lib -Wl,-rpath-link,$PREFIX/lib -L$PREFIX/lib -shared -Wl,-soname,librerun_sdk.so -o rerun_cpp/librerun_sdk.so rerun_cpp/CMakeFiles/rerun_sdk.dir/src/rerun/archetypes/annotation_context.cpp.o rerun_cpp/CMakeFiles/rerun_sdk.dir/src/rerun/archetypes/arrows2d.cpp.o rerun_cpp/CMakeFiles/rerun_sdk.dir/src/rerun/archetypes/arrows2d_ext.cpp.o rerun_cpp/CMakeFiles/rerun_sdk.dir/src/rerun/archetypes/arrows3d.cpp.o 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$PREFIX/lib/libarrow.so.1200.1.0  -lm -ldl -pthread && :
$BUILD_PREFIX/bin/../lib/gcc/x86_64-conda-linux-gnu/12.3.0/../../../../x86_64-conda-linux-gnu/bin/ld: cannot find $SRC_DIR/crates/rerun_c/../../target/release/librerun_c.a: No such file or directory
collect2: error: ld returned 1 exit status

It turned out that this was due to some rust activation script in conda-forge, that defined the CARGO_BUILD_TARGET env variable, and that apparently changed the location of the librerun_c.a library generated by cargo. To handle this, I modified the CMake code as reported in this PR. If the CARGO_BUILD_TARGET env variable is not defined, $ENV{CARGO_BUILD_TARGET} evalutes to an empty string, and the behaviour is exactly the same that you have before this PR.

I am not sure if it make sense to have this patch upstream, but I guess even to just ask that it was more efficient to open a PR rather than to open an issue to ask if it could make sense to open a PR. If you think it is better no to have this change upstream, feel free to close the PR, we can just keep the patch locally in https://github.com/conda-forge/librerun-sdk-feedstock/blob/main/recipe/fixrerun_c_location.patch .

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

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[Wumpf]

Makes sense to me, see not reason not to put this upstream in here.
Thanks for the continued effort on the conda-forge recipe!