Honor the same set of proxy environment variables (#10611)

Different parts of the project honor different sets of proxy environment
variables. With this commit all parts of the project will honor the same
set of proxy environment variables.

---------

Co-authored-by: Your Name <you@example.com>
Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems>

src/libcmd/network-proxy.hh

@@ -0,0 +1,22 @@
+#pragma once
+///@file
+
+#include "types.hh"
+
+namespace nix {
+
+/**
+ * Environment variables relating to network proxying. These are used by
+ * a few misc commands.
+ *
+ * See the Environment section of https://curl.se/docs/manpage.html for details.
+ */
+extern const StringSet networkProxyVariables;
+
+/**
+ * Heuristically check if there is a proxy connection by checking for defined
+ * proxy variables.
+ */
+bool haveNetworkProxyConnection();
+
+}