The `_` environment variable is a variable that is set by bash and some other
shells to point to the executable they use when executing a command. That is,
when executing `foo` from the command line, the shell sets `_` to
`/usr/bin/foo` (assuming that's where foo is).

However, nothing else does the same, so when e.g. a python program uses
`subprocess.Popen` to run another program, it doesn't set `_`. Worse, if that
python program itself was invoked from a shell, `_` would be set to e.g.
`/usr/bin/python3`.

So when nsis is invoked from a program that is not a shell, but the process
ancestry has a process that was a shell, `_` may be set to the first
intermediary program rather than nsis, which defeats nsis's assumption that `_`
would contain the nsis path. Ironically, nsis also has more reliable fallbacks
(using e.g.  /proc/self/exe), but somehow prefers `_`.

We remove the reliance of `_` entirely, for simplicity's sake.


diff -ruN nsis-3.07-src.orig/Source/util.cpp nsis-3.07-src/Source/util.cpp
--- nsis-3.07-src.orig/Source/util.cpp	2021-09-02 09:25:48.489016918 +0900
+++ nsis-3.07-src/Source/util.cpp	2021-09-02 09:26:21.158814484 +0900
@@ -810,10 +810,7 @@
   assert(rc == 0);
   return tstring(CtoTString(temp_buf));
 #else /* Linux/BSD/POSIX/etc */
-  const TCHAR *envpath = _tgetenv(_T("_"));
-  if (envpath)
-    return get_full_path(envpath);
-  else {
+  {
     char *path = NULL, *pathtmp;
     size_t len = 100;
     int nchars;