Add i386 to CI

[argilo]

Running VOLK's test suite on i386 revealed a broken protokernel and many broken tests:

• Allow for rounding error in float-to-int conversions #712
• Allow for rounding error in volk_32fc_s32f_magnitude_16i #713
• Remove broken sse_32 protokernels #714
• Fix flaky fm_detect test #715
• Fix flaky mod_range test #716
• Allow for rounding error in float-to-int interleave #718

Given this success, it would seem useful to add i386 to CI to catch more future bugs.

[jdemel]

Looking at the fixes, I agree.

[jdemel]

I'm looking for a way to run i386. A native GH Action would be preferable because they are much faster. I do have some hope that MacOS13 arm64 images do that for us on MacOS.

For non-x86, we use run-on-arch. This action doesn't provide an i386 image.

[argilo]

GNU Radio uses a 32-bit container: https://github.com/gnuradio/gnuradio/blob/bdaf5df2bd26325864d66bfb04caf8c8617296d1/.github/workflows/make-test.yml#L100-L102

[argilo]

Another way of catching floating-point bugs might be to use a tool like https://github.com/verificarlo/verificarlo.

But an i386 build still would have been needed to catch #714.