ci: implement robust multi-architecture build and test workflow

[rkt0209]

Overview

This PR introduces a comprehensive, multi-architecture GitHub Actions CI workflow (build.yml) for cups-filters. It ensures the repository automatically builds and passes its test suite across a matrix of different architectures, helping catch platform-specific regressions early.

Architectures Covered

The matrix strategy runs the build and test pipeline on four architectures:

• x86_64: Native (Ubuntu latest)
• arm64: Native (Ubuntu 24.04 ARM runner for optimal speed)
• armhf (armv7): Emulated via QEMU (uraimo/run-on-arch-action@v3)
• riscv64: Emulated via QEMU (uraimo/run-on-arch-action@v3)

Implementation Details & Fixes

Because cups-filters sits at the top of the OpenPrinting dependency chain, this workflow includes several critical configurations to ensure a stable testing environment:

• Complete Source Dependency Chain: Before building cups-filters, the CI dynamically clones, compiles, and installs pdfio, libcupsfilters, and libppd from source. This ensures we are always testing against the latest upstream code.
• System Conflict Prevention: Explicitly removes libcupsfilters-dev and libppd-dev from the runner to guarantee the build links against our fresh source builds, not stale Ubuntu system packages.
• Linker Cache Updates: Injects ldconfig immediately after installing each dependency so the system linker immediately locates the newly compiled .so files.
• Comprehensive Dependencies: Provides a complete list of required apt-get packages (including autopoint, libjxl-dev, libpoppler-cpp-dev, libexif-dev, and Avahi/DBus headers) to prevent ./autogen.sh and ./configure from failing.
• Strict QEMU Error Handling: Emulated run blocks are wrapped with set -ex at the very top of the script. This enforces fail-fast execution, ensuring that if a dependency fails to compile inside the emulator, the CI halts immediately rather than failing silently downstream.
• Failsafe Artifact Collection: The workflow unconditionally (if: always()) uploads all **/*.log files as GitHub Artifacts. This guarantees diagnostic data (like test-suite.log) is always available to developers, even if make check fails inside a nested directory.

Testing

All four architectures (native and emulated) have been tested on my fork and are successfully passing the autogen.sh, configure, make, and make check steps.