Pin action versions in OMB build workflow

Align with project conventions: pin docker/login-action to SHA
and update actions/checkout to v6.

Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Sam Barker <sam@quadrocket.co.uk>

Author: SamBarker

.github/workflows/build-omb-image.yml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           ref: ${{ inputs.kroxylicious_ref }}
 
@@ -74,7 +74,7 @@ jobs:
 
       - name: Log in to registry
         if: vars.REGISTRY_SERVER != ''
-        uses: docker/login-action@v3
+        uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3
         with:
           registry: ${{ vars.REGISTRY_SERVER }}
           username: ${{ vars.REGISTRY_USERNAME }}