Fix JRUBY-5858: "zsh:1 no matches found" when using gems loaded via Git in Bundler

* MRI unconditionally uses /bin/sh for shelling out on UNIX.

Author: headius

argparser.cpp

@@ -356,11 +356,7 @@ void ArgParser::prepareOptions() {
 #ifdef WIN32
     option += "cmd.exe";
 #else
-    const char* shell = getenv("SHELL");
-    if (shell == NULL) {
-        shell = "/bin/sh";
-    }
-    option += shell;
+    option += "/bin/sh";
 #endif
     javaOptions.push_back(option);