ci: migrate Windows cross compile to Ubuntu Focal #21122

We'd like to move the Windows cross compile in the CI to use Ubuntu Focal, to match the gitian descriptors that were just migrated to Focal.

I failed to do this as part of #21112. One issue seems to be using wine64, see logs here.

I think this is still racy and absent any other solutions, we'll have to remove the unit tests

Example failure:

Making check in src
make[1]: Entering directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
make[2]: Entering directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
make  check-TESTS check-local
make[3]: Entering directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
Running tests: addrman_tests from test/addrman_tests.cpp
Running tests: amount_tests from test/amount_tests.cpp
Running tests: arith_uint256_tests from test/arith_uint256_tests.cpp
make[4]: Entering directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
============================================================================
Testsuite summary for Bitcoin Core 21.99.0
============================================================================
# TOTAL: 0
# PASS:  0
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
============================================================================
make[4]: Leaving directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
Running tests: allocator_tests from test/allocator_tests.cpp
wine: created the configuration directory '/root/.wine'
wine: chdir to /tmp/wine-30FsIX/server-32-33c686 : No such file or directory
make[3]: *** [Makefile:16876: test/arith_uint256_tests.cpp.test] Error 1
make[3]: *** Waiting for unfinished jobs....
make[3]: Leaving directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
make[2]: *** [Makefile:15393: check-am] Error 2
make[2]: Leaving directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
make[1]: *** [Makefile:15078: check-recursive] Error 1
make[1]: Leaving directory '/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-w64-mingw32/src'
make: *** [Makefile:812: check-recursive] Error 1

https://cirrus-ci.com/task/6142530959966208?command=ci#L4523