Remove code path for deprecated `$ docker build`

[joonas-fi]

We have two different code paths:

1. modern $ docker buildx
2. legacy $ docker build

• this is deprecated

turbobob/cmd/bob/build.go

Lines 188 to 192 in 0e6e5bc

	// use buildx when platforms set. it's almost same as "$ docker build" but it almost transparently
	// supports cross-architecture builds via binftm_misc + QEMU userspace emulation
	useBuildx := len(dockerImage.Platforms) > 0
	if useBuildx {

Remove 2 so we only have one code path left.

[joonas-fi]

Might need to do this in GitHub actions to be fully automatic:

--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -18,8 +18,6 @@ jobs:
       # https://stackoverflow.com/a/59819441
       run: echo "git_sha_short=$(git rev-parse --short HEAD)" >> $GITHUB_OUTPUT

-    - uses: docker/setup-buildx-action@v3
-
     - name: Login to GitHub Container Registry
       uses: docker/login-action@v3
       with:
@@ -30,6 +28,9 @@ jobs:
     - name: Build
       run: |
         set -eu
+        docker run --privileged --rm tonistiigi/binfmt --install all
+        # https://docs.docker.com/build/builders/drivers/
+        docker buildx create --name builder --driver docker-container --use
         curl --fail --location --no-progress-meter --output /usr/local/bin/bob https://function61.com/go/turbobob-latest-stable-linux-amd64 && chmod +x /usr/local/bin/bob
         bob build in-ci-autodetect-settings