Revert #4832: Removed 32-bit MinGW job

[nulano]

Reverts #4832.
Update CI targets in installation.rst.

While working on #4890 I reread https://www.msys2.org/news/#2020-05-17-32-bit-msys2-no-longer-actively-supported and realized it is not referring to MSYS2 MinGW 32-bit, but the 32-bit version of MSYS2.

32-bit mingw-w64 packages are still supported, this is about the POSIX emulation layer, i.e. the runtime, Bash, MinTTY...

I.e. both MSYS2 MinGW 64-bit (mingw-w64-x86_64) and MSYS2 MinGW 32-bit (mingw-w64-i686) are still supported on 64-bit Windows.

After this date, we don't plan on building updated msys-i686 packages nor releasing i686 installers anymore. This is due to increasingly frustrating difficulties with limited 32-bit address space, high penetration of 64-bit systems and Cygwin (our upstream) starting their way to drop 32-bit support as well.

I.e. MSYS2 (the installer) is no longer supported on 32-bit Windows.

To clarify a bit, the MSYS2 subsystem is based on Cygwin, while the MinGW subsystems provide native Windows executables cross-compiled using MSYS2.

[hugovk]

Welcome back to the CI, MSYS2 MinGW 32-bit!