.github/zephyr: simplify and fix the git describe performance hack

Fixes the git describe/tag performance hack added in
commit 2328478 (".github/zephyr.yml: fix tags missing from `git -C
zephyr/ describe`") which worked for an amazingly long time (1.5 year)
but apparently ran its course. Git version 2.48 apparently does not like
it anymore. Replace it with something slower but simpler and safer.

Should fix build reproducibility issue #9797, much more details there.

Also fixes commit 4bc6488 (".github/zephyr: de-hardcode the name of
the zephyr remote")

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

Author: marc-hb

.github/workflows/zephyr.yml

@@ -179,14 +179,17 @@ jobs:
           # does not use --tags.
           #
           # 2. west fetches using the remote URL, not the remote name. So remote
-          # branches (if any) are missing from --decorate below
-          #
-          #  => an "empty" and quick fetch _with_ a refspec and the remote name fixes
-          #     both issues in no time.
+          # branches are missing from --decorate below. Cosmetic but annoying;
+          # especially in the "zmain" case.
 
           cd zephyr
-          time git fetch --filter=tree:0 "$(git remote |head -n1)" "$rem_rev":_branch_placeholder
-          git branch -D _branch_placeholder
+          # Fix problem 1. Indirectly fetches useless branches but seems quick enough
+          # thanks to the --filter
+          time git fetch --tags
+          # Fix problem 2.
+          _zurl=$(west list -f '{url}' zephyr)
+          git remote add sof_zep_rem "$_zurl"
+          time git fetch sof_zep_rem "$rem_rev"
 
           set -x
           west list
@@ -327,11 +330,16 @@ jobs:
           west init -l sof
           west update --narrow --fetch-opt=--filter=tree:0
 
-          # Get some tags to fix `git describe`, see build-linux comments above.
+          # Get some tags to fix `git describe` etc., see detailed build-linux comments above.
           cd zephyr
+          git fetch --tags
+
           $_rev = "$(git rev-parse HEAD)"
-          git fetch --filter=tree:0 "$(west list -f '{url}' zephyr)" "${_rev}:_branch_placeholder"
-          git branch -D _branch_placeholder
+          $_zurl = "$(west list -f '{url}' zephyr)"
+          git remote add sof_zep_rem "${_zurl}"
+          # Unlike Linux above, hardcode "main" for now. Will make no difference most
+          # of the time but keeps this bit consistent with Linux, tested and ready to use.
+          git fetch      sof_zep_rem main
 
 
       # Call Setup Python again to save the PIP packages in cache