Create tt-mlir-toolchain directory during the environment setup (#107)

[doc] Document the TOOLCHAIN dir dependency

Added the description in doc/src/build.md that the
`/opt/ttmlir-toolchain` directory needs to be created
before the environment setup cmake commands are executed.

And that the owner of the directory should be the user
that executes the cmake commands.

docs/src/build.md

@@ -15,6 +15,14 @@ cmake --build env/build
 
 > - It is recommended to use the **system installation of python3** for the virtual environment.
 >   Please ensure that you do not already have a venv activated before running the above command.
+> - Please ensure the directory `/opt/ttmlir-toolchain` exist and its
+>   owner is the current user, i.e. the one that executes the above `cmake` commands.
+>   The commands create it and assign the proper ownership are:
+>     ```bash
+>     sudo mkdir -p /opt/ttmlir-toolchain
+>     sudo chown -R $USER /opt/ttmlir-toolchain
+>     ```
+
 
 ## Build