jhbuild: Disable meson fallback subprojects

[philn]

These are time bombs. We should instead install missing build dependencies using apt-get.

[nikolaszimmermann]

Can you elaborate @philn?

[philn]

In GStreamer mono-repo by default if a build dependency is not found, a fallback to meson wrap files is attempted. Those wrap files specify the repo path and a revision among other things. If the revision is set to master or main we have no guarantee the build will succeed and there's a potential impact on reproducibility. Example, currently the wireplumber.wrap redirect looks like

[wrap-git]
url = https://gitlab.freedesktop.org/pipewire/wireplumber.git
revision = master

We should disable this fallback because of that.