ci(build.yaml): remove variable with image tag

Signed-off-by: Vojtěch Mareš <vojtech@mares.cz>

.woodpecker/build.yaml

@@ -5,7 +5,6 @@ variables:
   - &buildx-plugin "woodpeckerci/plugin-docker-buildx:6.0.2"
   - &repo "registry.mareshq.com/library/container-tools"
   - &platforms "linux/arm64/v8,linux/amd64"
-  - &image_tag "${CI_COMMIT_REF/ref\/heads\//}-${CI_COMMIT_SHA:0:8}"
 
 steps:
   build:
@@ -13,7 +12,7 @@ steps:
     settings:
       repo: *repo
       platforms: *platforms
-      tag: *image_tag
+      tag: ${CI_COMMIT_REF/ref/heads//}-${CI_COMMIT_SHA:0:8}
       logins:
         - registry: https://registry.mareshq.com
           username: