ext/svt.sh: Checkout SVT-AV1 v0.8.6

Syncs it with ext/svt.cmd

ext/svt.sh

diff --git a/ext/svt.sh b/ext/svt.sh
index 14bb930..c568d4c 100644
--- a/ext/svt.sh
+++ b/ext/svt.sh
@@ -2,7 +2,7 @@
 # then enable CMake's AVIF_CODEC_SVT and AVIF_LOCAL_SVT options.
 # cmake and ninja must be in your PATH.
 
-git clone --depth 1 https://gitlab.com/AOMediaCodec/SVT-AV1.git
+git clone -b v0.8.6 --depth 1 https://gitlab.com/AOMediaCodec/SVT-AV1.git
 
 cd SVT-AV1
 cd Build/linux