Fix msys2-runtime to respect an overriding HOME

[dscho]

Put into technical terms: Implement an env method for nsswitch.conf's db_home. We need this because Cygwin's windows method for the same setting does not detect flawed home directory settings (such as hidden UNC paths which e.g. ssh cannot handle).

[dscho]

See git-for-windows/msys2-runtime@develop...dscho:home-env

[dscho]

I just merged the PR, sadly without comment from people who objected violently to my first solution to the same problem.