CI: run the x86 PR build natively instead of under QEMU (#716)

The PR-validation build-docker-x86 job registers a QEMU binfmt handler
for linux/386 before building. i386 containers run natively on the
amd64 runners, but the explicit registration routes every binary in the
container through the qemu-i386 emulator, turning a ~15 minute build
into ~170 minutes. main.yml's build-linux-x86-docker job is identical
except it never installs QEMU, and it completes in 13-18 minutes.

Dropping the setup-qemu step makes the PR job match main.yml; the
--platform linux/386 image and build steps are unchanged.

.github/workflows/linux-compile.yml

@@ -152,11 +152,6 @@ jobs:
           fetch-tags: true
           submodules: recursive
 
-      - name: Set up QEMU
-        uses: docker/setup-qemu-action@v3
-        with:
-          platforms: linux/386
-
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3