CHANGE: Pin Linux bokken image to 4.50 (#1942)

fixes CI errors that came with 4.51

Author: stefanunity

.yamato/config.metadata

@@ -48,16 +48,16 @@ platforms_nix:
     installscript: unity-downloader-cli -c editor -c StandaloneSupport-IL2CPP --wait --fast -u
   - name: linux
     type: Unity::VM
-    image: package-ci/ubuntu-22.04:v4
+    image: package-ci/ubuntu-20.04:v4.50.0
     flavor: b1.large
   - name: linux_standalone
     type: Unity::VM
-    image: package-ci/ubuntu-22.04:v4
+    image: package-ci/ubuntu-20.04:v4.50.0
     flavor: b1.large
     runtime: StandaloneLinux64
   - name: linux_standalone_il2cpp
     type: Unity::VM
-    image: package-ci/ubuntu-22.04:v4
+    image: package-ci/ubuntu-20.04:v4.50.0
     flavor: b1.large
     runtime: StandaloneLinux64
     scripting-backend: Il2Cpp