FEAT: when $SHELL is not set on POSIX OS, /bin/sh is used as a default

Such a situation may happen for example on a clean Alpine Docker's image.

Author: Oldes

src/os/posix/host-lib.c

@@ -982,11 +982,7 @@ static inline REBOOL Open_Pipe_Fails(int pipefd[2]) {
 			const char ** argv_new = NULL;
 			sh = getenv("SHELL");
 			if (sh == NULL) {
-				int err = 2; /* shell does not exist */
-				if(write(info_pipe[W], &err, sizeof(err)) == -1) {
-					//nothing there... just to avoid "unused-result" compiler warning
-				}
-				exit(EXIT_FAILURE);
+				sh = "/bin/sh"; // if $SHELL is not defined
 			}
 			argv_new = OS_Make((argc + 3) * sizeof(char*));
 			argv_new[0] = sh;