installation: add mechanism for symlinking mlir-opt when creating venv (

#3660)

For the last couple of years, I've had a custom-built llvm bin folder in
my path. This has mostly been OK but sometimes resulted in weird build
errors. With this mechanism, we can have the best of both worlds, with
having a normal mlir-opt in the path except for when the venv is
activated:

```
~/Developer/xdslproject/xdsl sasha/misc/env *129 !1 ❯ which mlir-opt           
/opt/homebrew/opt/llvm/bin/mlir-opt
~/Developer/xdslproject/xdsl sasha/misc/env *129 ❯ uv run which mlir-opt  
/Users/sasha/Developer/xdslproject/xdsl/.venv/bin/mlir-opt
```

Makefile

@@ -29,6 +29,9 @@ uv-installed:
 .PHONY: ${VENV_DIR}/
 ${VENV_DIR}/: uv-installed
 	XDSL_VERSION_OVERRIDE="0+dynamic" uv sync ${VENV_EXTRAS}
+	@if [ ! -z "$(XDSL_MLIR_OPT_PATH)" ]; then \
+		ln -sf $(XDSL_MLIR_OPT_PATH) ${VENV_DIR}/bin/mlir-opt; \
+	fi
 
 # make sure `make venv` also works correctly
 .PHONY: venv

README.md

@@ -101,6 +101,12 @@ cd xdsl
 make venv
 ```
 
+To make a custom mlir-opt available in the virtual environment, set the `XDSL_MLIR_OPT_PATH` variable when running `make venv`, like so:
+
+``` bash
+XDSL_MLIR_OPT_PATH=/PATH/TO/LLVM/BUILD/bin/mlir-opt make venv
+```
+
 #### If you can't use `uv`
 
 For some systems and workflows, changing to a new dependency management system