firefox: Build with Python from Firefox

Remove `MACH_USE_SYSTEM_PYTHON=1` as our [MarIuX Python installation
does not satisfy the version requirements][1]:

    WARNING: You are using pip version 21.3.1; however, version 22.0.4 is available.
    You should consider upgrading via the '/scratch/local/tmp_ft1mxyo/bin/python -m pip install --upgrade pip' command.
    pygenometracks 3.6 requires pytest, which is not installed.
    requests-cache 0.8.1 has requirement attrs<22.0,>=21.2, but you have attrs 19.2.0.
    pygenometracks 3.6 has requirement matplotlib<=3.3.2,>=3.1.1, but you have matplotlib 3.4.3.
    cattrs 1.8.0 has requirement attrs>=20, but you have attrs 19.2.0.
    tensorflow 2.5.0 has requirement flatbuffers~=1.12.0, but you have flatbuffers 2.0.
    tensorflow 2.5.0 has requirement h5py~=3.1.0, but you have h5py 3.4.0.
    tensorflow 2.5.0 has requirement six~=1.15.0, but you have six 1.13.0.
    tensorflow 2.5.0 has requirement typing_extensions~=3.7.4, but you have typing-extensions 3.10.0.0.

[1]: mariux64/pkg-scripts#181

firefox.be0

@@ -39,7 +39,6 @@ mee_configure() {
 
 mee_build() {
     export CARGO_HOME=$B/.cargo
-    export MACH_USE_SYSTEM_PYTHON=1
     ${S}/mach build
 }