Remove msys2 hacks and vendor the whole thing

[pietroalbini]

On 2019-10-24 we had a CI outage due to the ca-certificates msys2 package being broken. The hack @mati865 implemented (:heart:) is to download an older, known good package beforehand and install it after msys2 updated its packages.

Once msys2/MINGW-packages#5887 is fixed we should revert the hack, but I think at this point it's worth investing in vendoring msys2 as a whole.

[crlf0710]

msys2 is using pacman and other tools from archlinux distro. Arch Linux provides per-day official repositories snapshots. User can "pin" to a specific date. I think it is possible to reuse that mechanism to vendoring msys2 as a whole, by providing a mirror of a specific date's version.

See here for more descriptions about it.

[mati865]

@crlf0710 that would be nice but msys doesn't remove old packages from their repo so the mirror would be enormous.
Search for mingw-w64-x86_64-qt5 in http://repo.msys2.org/mingw/x86_64, only those packages take dozens of GiB.

Also extracting it from archive would make CI a bit faster because you won't be modifying thousands of small files over and over. On Linux that is not a problem but pacman working through cygwin emulation a bit struggles on this part.