ci: fix cross-platform build and test failures

[zevorn]

Summary

• Add Homebrew bison and flex bin directories to GITHUB_PATH before
  configuring QEMU in the macOS build matrix.
• Replace POSIX-only pread()/pwrite() calls in the AX650X eMMC qtest with
  portable seek/read/write operations and open the raw image in binary mode.
• Reduce the existing 9p xattr limit test payload from 8 KiB to 1 KiB so the
  FID-limit test does not depend on the backing filesystem's xattr capacity.
• Disable documentation in the dedicated Clang 22 compiler build so unrelated
  Sphinx multiprocessing failures cannot mask compiler compatibility results.

Root causes

The macOS runners installed Homebrew bison, but bison is keg-only and was not
added to PATH. Meson therefore found /usr/bin/bison 2.3, while the Hexagon
build requires bison 3.0 or newer.

The MinGW64 and Windows CLANGARM64 builds failed because the AX650X qtest used
POSIX pread() and pwrite(), which are not declared on Windows. QEMU's osdep
layer already provides portable lseek(), regular file I/O, and binary-mode
flags for this use case.

The Linux ARM64 check failed the existing 9p local/xattr_limit/default test
with RLERROR/ENOSPC while clunking an 8 KiB xattr. The test is intended to
verify the number of outstanding xattr FIDs, not the host filesystem's maximum
xattr value size. A 1 KiB non-empty payload preserves all limit assertions and
works on constrained overlay/backing filesystems.

The Clang 22 job installed and configured the requested compiler successfully,
but its Sphinx -j auto worker exited with multiprocessing EOFError while
Ninja was compiling in parallel. Documentation is already covered elsewhere;
keeping this dedicated job compiler-only removes the unrelated resource race.

Original failing jobs:

• macOS ARM
• macOS Intel
• MinGW64 cross
• Windows CLANGARM64
• Windows UCRT64
• Windows CLANG64
• Windows MINGW64
• Linux ARM64 check
• Linux x86_64 check
• Clang 22 Sphinx failure

Validation

• git diff --check
• actionlint -shellcheck= .github/workflows/build.yml
• scripts/checkpatch.pl --branch HEAD^..HEAD
• Verified on macOS ARM that the workflow path resolves bison 3.8.2 instead of
  the system bison 2.3.
• Native AArch64 build plus
  qemu:qtest-aarch64/ax650x-pyramid-test: 9/9 subtests passed.
• Native ARM64 Ubuntu 24.04 container build plus the exact 9p
  local/xattr_limit/default test path: 1/1 passed.

The original devel matrix and the updated fix-branch matrix remain under
observation for additional independent failures.