Fix for postprocessing scripts not working on UNIX when $SHELL is undefined (#4101)

Fix for PP scripts not working on UNIX when $SHELL is undefined

thanks @jfbauer432 for pointing the problem out.
thanks @lukasmatena for fixing it in PrusaSlicer (commit 87a5116)

Co-authored-by: Lukáš Matěna <lukasmatena@seznam.cz>

src/libslic3r/GCode/PostProcessor.cpp

@@ -157,7 +157,7 @@ static int run_script(const std::string &script, const std::string &gcode, std::
 {
     // Try to obtain user's default shell
     const char *shell = ::getenv("SHELL");
-    if (shell == nullptr) { shell = "sh"; }
+    if (shell == nullptr) { shell = "/bin/sh"; }
 
     // Quote and escape the gcode path argument
     std::string command { script };