.github/zephyr: test new fancy --filter=tree:0 cloning option

Test new, fancy and impressive looking git --filter optimization on
non-critical "manifest check". Extend it later to other checks if it
hasn't caused any issue.

https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/
zephyrproject-rtos/west#638 (comment)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

.github/workflows/zephyr.yml

@@ -37,7 +37,7 @@ jobs:
           pip3 install west
           cd workspace/sof/
           west init -l
-          west update
+          west update --fetch-opt=--filter=tree:0
 
   build-linux:
     runs-on: ubuntu-20.04