docs: update README to include pixi instructions for running tests

Author: cpcloud

README.md

@@ -37,12 +37,19 @@ they are compiled for the appropriate compute capability.
 ```
 cd testing
 # Optionally, build test binaries and point to their location for the test suite
-make
+make -j $(nproc)
 export NUMBA_CUDA_TEST_BIN_DIR=`pwd`
 # Execute tests
 pytest -n auto -v
 ```
 
+Alternatively, you can use [pixi](https://pixi.sh/latest/installation/) to wrap all of that up for you:
+
+```
+# run tests against CUDA 13
+pixi run -e cu13 test -n auto -v
+```
+
 
 Testing should discover the `numba.cuda` module from the `numba_cuda` package. You
 can check where `numba.cuda` files are being located by running