Build.sh's mechanism for detecting/installing meson does not work under ubuntu 24.04

[theunkn0wn1]

Do not report issue for old MangoHud versions

Describe the bug Build.sh is hardcoded to detect meson as installed in the global python3 installation. If it is not detected, it unconditionally calls pip3 install meson, which always fails under ubuntu 24.04

Note: for ubuntu 24.04.1 python3 -m pip install calls will always fail, as Ubuntu decided we are not allowed to install packages by these means

Build.sh should simply detect the presense of meson via the environment, instead of assuming its location.

List relevant hardware/software information

• Ubuntu 24.04.1 LTS

To Reproduce Steps to reproduce the behavior:

1. clone flightlessmango/MangoHud
2. ./build.sh install

Expected behavior Build script should detect the presence of meson on the path, and use it.

Screenshots N/A Additional context

$ ./build.sh install                                                                                                                                                               [14:22:50]
Command: install 
# Checking dependencies for "ubuntu"
# Missing dependencies:g++-multilib mesa-common-dev libxnvctrl-dev libxkbcommon-dev:i386 libwayland-dev:i386 
Do you wish the script to install these packages? [y/N]n
Continuing with missing dependencies
WARNING: Package(s) not found: meson
error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.

    If you wish to install a non-Debian-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
    sure you have python3-full installed.

    If you wish to install a non-Debian packaged Python application,
    it may be easiest to use pipx install xyz, which will manage a
    virtual environment for you. Make sure you have pipx installed.

    See /usr/share/doc/python3.12/README.venv for more information.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.
FAIL: 1