tremor: move to a tarball snapshot

tremor is the only svn-based project we have left and has a commit
very rarely (2 commits to the code in the past 3 years according to
https://git.xiph.org/?p=tremor.git).
7 jobs for no-svn in 63 minutes and 37 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #99442
manifest

00:00:46

 
  Build
passed #99443
cerbero deps fedora x86_64

00:50:47

passed #99444
cerbero deps windows x86

00:24:59

passed #99445
cerbero deps windows x86_64

00:47:50

 
  Full Builds
passed #99447
build cerbero cross win32

00:09:54

passed #99448
build cerbero cross win64

00:06:14

passed #99446
build cerbero fedora x86_64

00:07:58