Flaky CI jobs

[fredrik-johansson]

• Regression check sometimes fails on fallback code on runners where assembly isn't available
• The MSVC job often fails installing vcpkg packages
• CI / ASan, BLAS, examples sometimes fails on the mfcoefs example program. I've been unable to reproduce this locally but it happens often enough in CI runs that it should be possible to track down and fix.

[albinahlback]

The vcpkg issue seems to be recent: microsoft/vcpkg#53394

[fredrik-johansson]

This one for Regression check is trivial to fix:

/home/runner/work/flint/flint/src/mpn_extras/sqrhigh_basecase.c: In function ‘flint_mpn_sqrhigh_2’:
  CC  mpn_extras/sqrhigh_basecase.c
/home/runner/work/flint/flint/src/mpn_extras/sqrhigh_basecase.c:46:15: error: variable ‘b’ set but not used [-Werror=unused-but-set-variable]
   46 |     mp_limb_t b, low;
      |               ^
cc1: all warnings being treated as errors
make: *** [Makefile:658: build/mpn_extras/sqrhigh_basecase.lo] Error 1
make: *** Waiting for unfinished jobs....