Fix Nix to properly work with stores using a scoped IPv6 address

According to RFC4007[1], IPv6 addresses can have a so-called zone_id
separated from the actual address with `%` as delimiter. In contrast to
Nix 2.3, the version on `master` doesn't recognize it as such:

    $ nix ping-store --store ssh://root@fe80::1%18 --experimental-features nix-command
    warning: 'ping-store' is a deprecated alias for 'store ping'
    error: --- Error ----------------------------------------------------------------- nix
    don't know how to open Nix store 'ssh://root@fe80::1%18'

I modified the IPv6 match-regex accordingly to optionally detect this
part of the address. As we don't seem to do anything special with it, I
decided to leave it as part of the URL for now.

Fixes #4490

[1] https://tools.ietf.org/html/rfc4007

src/libutil/tests/url.cc

@@ -117,6 +117,24 @@ namespace nix {
         ASSERT_EQ(parsed, expected);
     }
 
+    TEST(parseURL, parseScopedRFC4007IPv6Address) {
+        auto s = "http://[fe80::818c:da4d:8975:415c\%enp0s25]:8080";
+        auto parsed = parseURL(s);
+
+        ParsedURL expected {
+            .url = "http://[fe80::818c:da4d:8975:415c\%enp0s25]:8080",
+            .base = "http://[fe80::818c:da4d:8975:415c\%enp0s25]:8080",
+            .scheme = "http",
+            .authority = "[fe80::818c:da4d:8975:415c\%enp0s25]:8080",
+            .path = "",
+            .query = (StringMap) { },
+            .fragment = "",
+        };
+
+        ASSERT_EQ(parsed, expected);
+
+    }
+
     TEST(parseURL, parseIPv6Address) {
         auto s = "http://[2a02:8071:8192:c100:311d:192d:81ac:11ea]:8080";
         auto parsed = parseURL(s);

src/libutil/url-parts.hh

@@ -8,7 +8,7 @@ namespace nix {
 // URI stuff.
 const static std::string pctEncoded = "(?:%[0-9a-fA-F][0-9a-fA-F])";
 const static std::string schemeRegex = "(?:[a-z][a-z0-9+.-]*)";
-const static std::string ipv6AddressSegmentRegex = "[0-9a-fA-F:]+";
+const static std::string ipv6AddressSegmentRegex = "[0-9a-fA-F:]+(?:%\\w+)?";
 const static std::string ipv6AddressRegex = "(?:\\[" + ipv6AddressSegmentRegex + "\\]|" + ipv6AddressSegmentRegex + ")";
 const static std::string unreservedRegex = "(?:[a-zA-Z0-9-._~])";
 const static std::string subdelimsRegex = "(?:[!$&'\"()*+,;=])";