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.cc

@@ -32,4 +32,17 @@ std::map<std::string, std::string> getEnv()
     return env;
 }
 
+void clearEnv()
+{
+    for (auto & name : getEnv())
+        unsetenv(name.first.c_str());
+}
+
+void replaceEnv(const std::map<std::string, std::string> & newEnv)
+{
+    clearEnv();
+    for (auto & newEnvVar : newEnv)
+        setEnv(newEnvVar.first.c_str(), newEnvVar.second.c_str());
+}
+
 }