fix shell detection

[mo271]

Attempt to fix #3564

[mo271]

The idea with this regular expression is to also match dash sh, zsh and so on...

[Patterner]

it works, but i'm not really liking it. it's way too vague. and i'm not really sure why this there.

i would've done something like
^/(/usr)?/bin/env .*sh)|(/usr/)?/bin/.*sh$
i.e /usr/bin/env dash /bin/env zsh /bin/zsh /usr/bin/fish

*sh matches ssh and texhash

if it's MSYS2 deal with it otherwise just use "a2x" as it is.

[eustas]

Provided regexp also matches /usr/bin/ssh and /usr/bin/env texhash. So let's keep it simple, and rely on sanity of package developers.