gitlab/cerbero: Don't fail when there's no macOS cache

`test -d "${CERBERO_HOME}/dist/${ARCH}"` will cause the build to error
out if the dist dir doesn't exist, which will happen if there's no
cache, for example:
https://gitlab.freedesktop.org/seungha.yang/gst-plugins-bad/-/jobs/7405565

What we actually want to do, is to not perform the rsync if there's no
cache, since it means we want to rebuild from scratch; for instance,
if the current MR uses a custom cerbero branch of the same name which
won't have a prebuilt cache.

Part-of: <!398>
41 jobs for master in 33 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build Docker
passed #7407650
alpine amd64 manifest builder docker

00:00:13

passed #7407649
android docker

00:00:17

passed #7407648
cerbero fedora amd64 docker

00:00:12

passed #7407647
fedora amd64 docker

00:00:12

passed #7407651
gst-indent amd64 docker

00:00:17

passed #7407652
test manifest amd64 docker

00:00:13

 
  Preparation
passed #7407654
gst indent

00:00:12

manual #7407653
manual
manifest
passed #7407655
test manifest

00:00:15

 
  Build
created #7407664
gstreamer
build cerbero cross-android universal
created #7407668
gst-ios-14.3
build cerbero cross-ios universal
created #7407665
gstreamer
build cerbero cross win32
created #7407666
gstreamer
build cerbero cross win64
created #7407663
gstreamer
build cerbero fedora x86_64
created #7407667
gst-macos-11.1
build cerbero macos x86_64
created #7407671
docker windows gstreamer-windows 1809
build cerbero msvc x86_64
created #7407659
build clang fedora x86_64
created #7407670
build gst-omx tizonia fedora x86_64
created #7407669
build gst-omx zynq fedora x86_64
created #7407662
docker windows 1809 allowed to fail manual
build msys2
created #7407656
build nodebug fedora x86_64
created #7407657
build static fedora x86_64
created #7407658
build static nodebug fedora x86_64
created #7407660
docker windows 1809
build vs2017 amd64
created #7407661
docker windows 1809
build vs2017 x86
 
  Test
created #7407672
gstreamer
check fedora
created #7407673
gstreamer
integration testsuites fedora 1/4
created #7407674
gstreamer
integration testsuites fedora 2/4
created #7407675
gstreamer
integration testsuites fedora 3/4
created #7407676
gstreamer
integration testsuites fedora 4/4
created #7407681
gstreamer
valgrind bad
created #7407678
gstreamer
valgrind base
created #7407677
gstreamer
valgrind core
created #7407682
gstreamer
valgrind ges
created #7407679
gstreamer
valgrind good
created #7407680
gstreamer
valgrind ugly
 
  Integrate
created #7407685
cross-android universal examples
created #7407687
gst-ios-14.3
cross-ios universal examples
created #7407686
documentation
created #7407683
gstreamer-full
created #7407684
gstreamer-full-minimal