chore: remove references to installing cargo-ament-build via cargo

[esteve]

Now that cargo-ament-build is avaialble via PyPI as a Python wheel and that cargo-ros-cargo depends on it, we can install both in one go.

[luca-della-vedova]

Ah this all happened while I was sleeping 😅

I guess this is OK for now but I would have argued that we will actually need to do the opposite soon, since installing pip packages in Jazzy / 24.04 is very painful, and it is unclear whether we will be able to package cargo-ament-build as a debian package.
I suspect the final intermediate solution will be to revert this and actually install cargo-ament-build from cargo and let apt manage the colcon extensions instead

[esteve]

I don't think that would be necessary, the cargo-ament-build Python wheel could be distributed as a Debian package as is, as a precompiled binary and we wouldn't need to worry about the intermediate solution you describe.

For me, the ideal solution would be to mimic what the ruff Debian package does.

[mxgrey]

If the precompiled binary can be distributed as-is through debian then that's definitely the best case scenario.