alpine,debian: fix tests/modules race on non-module arches; make pip conditional in Alpine

[mike-golant]

Summary

• Avoid tests/modules race on non-module arches by limiting parallelism
  • Introduces MAKE_JOBS: amd64/arm64 keep full parallelism (-j nproc); all others use -j1
  • Fixes intermittent ld: cannot find commandfilter.xo (seen on arm/v6,v7 and other slow/emu arches)
• Alpine: call pip only when BUILD_WITH_MODULES=yes and use python3 -m pip

Verification (local, via colima + buildx + binfmt)

• Alpine: linux/arm/v6 — OK
• Alpine: linux/arm/v7 — OK
• Debian: linux/arm/v7 — OK

Notes

• This addresses failures that varied across OS/arch due to timing; serializing test module build removes the race without penalizing amd64/arm64 builds where modules are enabled.