CheckEnv should not use $SHELL

[jscheel]

$SHELL is a environment variable to specify your preferred shell, not the current running shell (http:https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03). This can cause all sorts of issues if your scripts are written in a different shell than your preferred shell (e.g. writing everything as bash scripts, but using fish for yourself).