Clean up env var logic in preparation for Windows

It's a little weird we don't check the return status for these, but
changing that would introduce risk so I did not.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>

src/libutil/environment-variables.hh

@@ -28,6 +28,14 @@ std::optional<std::string> getEnvNonEmpty(const std::string & key);
  */
 std::map<std::string, std::string> getEnv();
 
+/**
+ * Like POSIX `setenv`, but always overrides.
+ *
+ * We don't need the non-overriding version, and this is easier to
+ * reimplement on Windows.
+ */
+int setEnv(const char * name, const char * value);
+
 /**
  * Clear the environment.
  */