Pin to buildkit 0.9.1

docker/build-push-action#498 (comment)
docker/buildx#834
moby/buildkit#2461

Looks like a fix is merged into buildkit, but we have to wait for
a new release to get the fix.

.github/workflows/test-build.yml

@@ -93,6 +93,8 @@ jobs:
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
+        with:
+          driver-opts: image=moby/buildkit:v0.9.1
 
       - name: Login to GitHub Container Registry
         uses: docker/login-action@v1
@@ -146,6 +148,8 @@ jobs:
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
+        with:
+          driver-opts: image=moby/buildkit:v0.9.1
 
       - name: Login to GitHub Container Registry
         uses: docker/login-action@v1
@@ -224,6 +228,8 @@ jobs:
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1
+        with:
+          driver-opts: image=moby/buildkit:v0.9.1
 
       - name: Login to GitHub Container Registry
         uses: docker/login-action@v1