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.

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