cerbero ci: Wipe in before_script because gitlab doesn't

Noticed in this job, which failed because we rebuilt on top of an
existing builddir:

https://gitlab.freedesktop.org/nirbheek/cerbero/-/jobs/4772728

Part-of: <!358>
40 jobs for master in 45 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build Docker
passed alpine amd64 manifest builder docker #4779772

00:00:13

passed android docker #4779771

00:00:19

passed cerbero fedora amd64 docker #4779770

00:00:33

passed fedora amd64 docker #4779769

00:00:34

passed gst-indent amd64 docker #4779773

00:00:33

passed test manifest amd64 docker #4779774

00:00:19

 
  Preparation
passed gst indent #4779776

00:00:08

manual manifest #4779775
manual
passed test manifest #4779777

00:00:11

 
  Build
created build cerbero cross-android universal #4779786
gstreamer
created build cerbero cross-ios universal #4779790
gst-ios-13.2
created build cerbero cross win32 #4779787
gstreamer
created build cerbero cross win64 #4779788
gstreamer
created build cerbero fedora x86_64 #4779785
gstreamer
created build cerbero macos x86_64 #4779789
gst-macos-10.15
created build cerbero msvc x86_64 #4779793
docker windows gstreamer-windows 1809 allowed to fail
created build clang fedora x86_64 #4779781
created build gst-omx tizonia fedora x86_64 #4779792
created build gst-omx zynq fedora x86_64 #4779791
created build msys2 #4779784
docker windows 1809 allowed to fail manual
created build nodebug fedora x86_64 #4779778
created build static fedora x86_64 #4779779
created build static nodebug fedora x86_64 #4779780
created build vs2017 amd64 #4779782
docker windows 1809
created build vs2017 x86 #4779783
docker windows 1809
 
  Test
created check fedora #4779794
gstreamer
created integration testsuites fedora 1/4 #4779795
gstreamer
created integration testsuites fedora 2/4 #4779796
gstreamer
created integration testsuites fedora 3/4 #4779797
gstreamer
created integration testsuites fedora 4/4 #4779798
gstreamer
created valgrind bad #4779803
gstreamer
created valgrind base #4779800
gstreamer
created valgrind core #4779799
gstreamer
created valgrind ges #4779804
gstreamer
created valgrind good #4779801
gstreamer
created valgrind ugly #4779802
gstreamer
 
  Integrate
created cross-android universal examples #4779806
created cross-ios universal examples #4779808
gst-ios-13.2
created documentation #4779807
created gstreamer-full #4779805