ci: pin meson version to previous release

When 0.56.0 was released it got auto-updated
and broke the build due to various bugs.

Roll back to 0.55.3 for the time being.
7 jobs for alatiera/pin-meson in 4 minutes and 46 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #5430629
android fedora arm64

00:04:45

passed #5430623
debian sid

00:02:48

passed #5430628
gst-ios-13.2
ios arm64

00:00:37

passed #5430627
gst-macos-10.15
macos x86_64

00:00:41

manual #5430626
gstreamer-windows 1809 windows docker allowed to fail manual
msys2
passed #5430624
gstreamer-windows 1809 windows docker
vs2017 amd64

00:00:53

passed #5430625
gstreamer-windows 1809 windows docker
vs2017 x86

00:00:52