e_start - fix alternate ifdef path putenv memory issue

we'd have corrupted env vars with the alloca code we had to store the
env var, so always malloc it at all times. as we won't (likely) be
calling env_set() multiple times for the same environment we won't be
leaking, and at worst - not very much at all.

@fix

src/bin/e_start_main.c

@@ -75,10 +75,13 @@ env_set(const char *var, const char *val)
         char *buf;
         size_t size = strlen(var) + 1 + strlen(val) + 1;
 
-        buf = alloca(size);
+        // yes - this does leak. we know. intentional if we set the same
+        // env multiple times because we likely won't and alloca is wrong
+        // as we lose the env content on the stack. but becomes part of
+        // the environment so it has to stay around after putenv is called
+        buf = malloc(size);
         sprintf(buf, "%s=%s", var, val);
-        if (getenv(var)) putenv(buf);
-        else putenv(strdup(buf));
+        putenv(buf);
 #endif
      }
    else