syntax: sh: let the header regex match also busybox shell scripts

And recognize calls via 'env' too.

This addresses https://savannah.gnu.org/bugs/?54674.
Reported-by: Aaron Sears <geodelic@gmail.com>

syntax/sh.nanorc

@@ -1,7 +1,7 @@
 ## Here is an example for Bourne shell scripts.
 
 syntax sh "(\.sh|\.(a|ba|c|da|k|mk|pdk|tc|z)(sh|shrc|sh_profile)|/(etc/|\.)profile)$"
-header "^#!.*/(((a|ba|c|da|k|mk|pdk|tc|z)?sh)|openrc-run|runscript)"
+header "^#!.*/(((a|ba|c|da|k|mk|pdk|tc|z)?sh)|(busybox|env) +sh|openrc-run|runscript)"
 magic "(POSIX|Bourne-Again) shell script.*text"
 linter dash -n
 comment "#"