Fix python3 executable path

[Ryanf55]

Fixes build errors encountered in Dockerized environment.

We now use the FindPython3 module.
Python2, and the path to /usr/bin/python is not installed on Ubuntu 22 or later.

I encountered build problems here: mavlink/MAVSDK#2691 (comment)

Tested with this dockerfile:

FROM ubuntu:22.04

WORKDIR /ws

# https://mavsdk.mavlink.io/main/en/cpp/guide/build_linux.html

RUN --mount=target=/var/lib/apt/lists,type=cache,sharing=locked \
    --mount=target=/var/cache/apt,type=cache,sharing=locked \
    apt-get update && \
    DEBIAN_FRONTEND=noninteractive apt-get -y install \
        build-essential \
        cmake \
        git \
        python3 \
        python3-pip

COPY . .
RUN python3 -m pip install -r requirements.txt
RUN cmake -B build -S libs/cpp
RUN cmake --build build
RUN cmake --install build --prefix install

I am not sure how/why CI is currently working. Maybe it isn't on Ubuntu:22.

[hamishwillee]

@bkueng When you get a moment!

[julianoes]

I think that's ok but I'll leave it up to @bkueng.

[bkueng]

Looks good, thanks. You might want to disable testing if you just want to install it.

CI runs on ubuntu-latest: https://github.com/mavlink/libevents/blob/main/.github/workflows/tests.yml#L17