mingw on AppVeyor failing - can we do anything about it?

[tobias-loew]

Currently the mingw builds on AppVeyor are failing. The reason is that the package manager tries to download old images which seem no longer available (cf. below or https://ci.appveyor.com/project/tobias-loew/unordered/builds/39210079/job/pon5fdeju46656t9)
E.g. the oldest version on http://repo.msys2.org/mingw/x86_64/ is mingw-w64-x86_64-icu-66.1-1-any.
Can we do anything about it?

C:\boost-root>FOR %a IN ("gcc" "icu" "libiconv" "openssl" "xz" "zlib") DO (c:\msys64\usr\bin\env MSYSTEM=MINGW32                           c:\msys64\usr\bin\bash -l -c       "pacman --sync --needed --noconfirm mingw32/mingw-w64-i686-%a"   || EXIT /B 1 ) 
C:\boost-root>(c:\msys64\usr\bin\env MSYSTEM=MINGW32                           c:\msys64\usr\bin\bash -l -c       "pacman --sync --needed --noconfirm mingw32/mingw-w64-i686-"gcc""   || EXIT /B 1 ) 
warning: mingw-w64-i686-gcc-9.1.0-3 is up to date -- skipping
 there is nothing to do
C:\boost-root>(c:\msys64\usr\bin\env MSYSTEM=MINGW32                           c:\msys64\usr\bin\bash -l -c       "pacman --sync --needed --noconfirm mingw32/mingw-w64-i686-"icu""   || EXIT /B 1 ) 
resolving dependencies...
looking for conflicting packages...
Packages (1) mingw-w64-i686-icu-64.2-1
Total Download Size:   17.45 MiB
Total Installed Size:  93.00 MiB
:: Proceed with installation? [Y/n] warning: no /var/cache/pacman/pkg/ cache exists, creating...
:: Retrieving packages...
error: failed retrieving file 'mingw-w64-i686-icu-64.2-1-any.pkg.tar.xz' from repo.msys2.org : The requested URL returned error: 404
error: failed retrieving file 'mingw-w64-i686-icu-64.2-1-any.pkg.tar.xz' from sourceforge.net : The requested URL returned error: 404
error: failed retrieving file 'mingw-w64-i686-icu-64.2-1-any.pkg.tar.xz' from www2.futureware.at : The requested URL returned error: 404
error: failed retrieving file 'mingw-w64-i686-icu-64.2-1-any.pkg.tar.xz' from mirror.yandex.ru : The requested URL returned error: 404
warning: failed to retrieve some files
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.
C:\boost-root\libs\unordered>set lastexitcode=1 
C:\boost-root\libs\unordered>set  1>C:\Users\appveyor\AppData\Local\Temp\1\tmp6BA.tmp 
C:\boost-root\libs\unordered>echo C:\boost-root\libs\unordered  1>C:\Users\appveyor\AppData\Local\Temp\1\tmp6BB.tmp 
C:\boost-root\libs\unordered>exit /b 1 
Command exited with code 1

[Flamefire]

Fixed by #77