Update buildkit CI after buildkit helm chart image change

[Secretions]

The previous PR (#246) updated how the helm chart references the buildkit images, but our build process to vendor the buildkit images uses that chart's values, using the same string substitution separately, to determine the base image. This causes us to use the incorrect image when vendoring buildkit.

[Secretions]

CI skips buildkit in PRs here, so I ran it locally like this:

[anglerfish : 0 : 3:00PM] buildkit>docker buildx build --target root --build-arg BUILDKIT_TAG=$(yq '.buildkit.image.tag' ../../deployments/helm/hephaestus/values.yaml) . -t buildkit:root
[+] Building 0.8s (6/6) FINISHED                                                                               docker:desktop-linux
 => [internal] load build definition from Dockerfile                                                                           0.0s
 => => transferring dockerfile: 879B                                                                                           0.0s
 => [internal] load metadata for docker.io/moby/buildkit:v0.16.0                                                               0.7s
 => [internal] load .dockerignore                                                                                              0.0s
 => => transferring context: 2B                                                                                                0.0s
 => [root 1/2] FROM docker.io/moby/buildkit:v0.16.0@sha256:bc1fe18224dbcb92599139db0c745696c48ba9fd4ac24038d1fa81fdd7dcac27    0.0s
 => CACHED [root 2/2] RUN apk update &&     apk upgrade &&     rm -rf /var/cache/apk                                           0.0s
 => exporting to image                                                                                                         0.0s
 => => exporting layers                                                                                                        0.0s
 => => writing image sha256:92d43ebbf835f0cd925f6abf69f021bb455f831837a5630098955cd76d3d400c                                   0.0s
 => => naming to docker.io/library/buildkit:root                                                                               0.0s
[anglerfish : 0 : 3:00PM] buildkit>docker buildx build --target rootless --build-arg BUILDKIT_TAG=$(yq '.buildkit.rootlessImage.tag' ../../deployments/helm/hephaestus/values.yaml) . -t buildkit:rootless
[+] Building 0.4s (8/8) FINISHED                                                                               docker:desktop-linux
 => [internal] load build definition from Dockerfile                                                                           0.0s
 => => transferring dockerfile: 879B                                                                                           0.0s
 => [internal] load metadata for docker.io/moby/buildkit:v0.16.0-rootless                                                      0.4s
 => [internal] load .dockerignore                                                                                              0.0s
 => => transferring context: 2B                                                                                                0.0s
 => [rootless 1/4] FROM docker.io/moby/buildkit:v0.16.0-rootless@sha256:fed5f1fd2f8c2b246bc227e3f72953981dbe163a09add4c8cb9cc  0.0s
 => CACHED [rootless 2/4] RUN apk update &&     apk upgrade &&     rm -rf /var/cache/apk                                       0.0s
 => CACHED [rootless 3/4] RUN deluser user &&     adduser -u 1000 -D user &&     chown -R user:user /home/user &&     mkdir -  0.0s
 => CACHED [rootless 4/4] RUN addgroup -S -g 1337 istio &&     addgroup user istio &&     echo user:100000:150000 | tee /etc/  0.0s
 => exporting to image                                                                                                         0.0s
 => => exporting layers                                                                                                        0.0s
 => => writing image sha256:29cc737b56d4e2bcb8d5e90ece0f3e5aa12a48e666bb53d856189e914b0de001                                   0.0s
 => => naming to docker.io/library/buildkit:rootless