[build] fix CI warnings issued by "git describe" (sonic-net#13098)

./functions.sh script contains twice the command "git describe --tags".
Azure pipelines currently retrieves the repository content by using a
"git clone" or "git fetch" command with the option "--depth".
When the depth is not enough sized to reach a tag in the "git log"
history,  "git describe --tags" generates on stderr the warning:
"fatal: No names found, cannot describe anything."

https://stackoverflow.com/questions/4916492/git-describe-fails-with-fatal-no-names-found-cannot-describe-anything

During the build process, the functions.sh script is a lot consumed and
this pointless warning appears in CI thousand lines w/o any added value.

This situation can be easily avoided by redirecting related commands
stderr output to /dev/null.

Signed-off-by: Guillaume Lambert <[email protected]>

functions.sh

@@ -51,8 +51,8 @@ docker_try_rmi() {
 }
 
 sonic_get_version() {
-    local describe=$(git describe --tags)
-    local latest_tag=$(git describe --tags --abbrev=0)
+    local describe=$(git describe --tags 2>/dev/null)
+    local latest_tag=$(git describe --tags --abbrev=0 2>/dev/null)
     local branch_name=$(git rev-parse --abbrev-ref HEAD)
     if [ -n "$(git status --untracked-files=no -s --ignore-submodules)" ]; then
         local dirty="-dirty-$BUILD_TIMESTAMP"