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.cc

@@ -0,0 +1,45 @@
+#include "network-proxy.hh"
+
+#include <algorithm>
+#include <boost/algorithm/string.hpp>
+
+#include "environment-variables.hh"
+
+namespace nix {
+
+static const StringSet lowercaseVariables{"http_proxy", "https_proxy", "ftp_proxy", "all_proxy", "no_proxy"};
+
+static StringSet getAllVariables()
+{
+    StringSet variables = lowercaseVariables;
+    for (const auto & variable : lowercaseVariables) {
+        variables.insert(boost::to_upper_copy(variable));
+    }
+    return variables;
+}
+
+const StringSet networkProxyVariables = getAllVariables();
+
+static StringSet getExcludingNoProxyVariables()
+{
+    static const StringSet excludeVariables{"no_proxy", "NO_PROXY"};
+    StringSet variables;
+    std::set_difference(
+        networkProxyVariables.begin(), networkProxyVariables.end(), excludeVariables.begin(), excludeVariables.end(),
+        std::inserter(variables, variables.begin()));
+    return variables;
+}
+
+static const StringSet excludingNoProxyVariables = getExcludingNoProxyVariables();
+
+bool haveNetworkProxyConnection()
+{
+    for (const auto & variable : excludingNoProxyVariables) {
+        if (getEnv(variable).has_value()) {
+            return true;
+        }
+    }
+    return false;
+}
+
+}