chore(deps): update docker/setup-buildx-action action to v3.12.0

Author: renovate[bot]

.github/workflows/build-testflinger-testenv-container.yml

@@ -31,7 +31,7 @@ jobs:
 
     steps:
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@e468171a9de216ec08956ac3ada2f0791b6bd435 # v3.11.1
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
         with:
           cache-binary: false
           config-inline: |

.github/workflows/server-charm-release.yml

@@ -39,7 +39,7 @@ jobs:
 
     steps:
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@e468171a9de216ec08956ac3ada2f0791b6bd435 # v3.11.1
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
         with:
           cache-binary: false
           config-inline: |