Add priority setting to stores

This allows overriding the priority of substituters, e.g.

  $ nix-store --store ~/my-nix/ -r /nix/store/df3m4da96d84ljzxx4mygfshm1p0r2n3-geeqie-1.4 \
    --substituters 'http://cache.nixos.org?priority=100 daemon?priority=10'

Fixes #3264.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstore/globals.cc b/src/libstore/globals.cc
index 6fedf3d56..cec85edca 100644
--- a/src/libstore/globals.cc
+++ b/src/libstore/globals.cc
@@ -127,7 +127,7 @@ template<> void BaseSetting<SandboxMode>::set(const std::string & str)
     else throw UsageError("option '%s' has invalid value '%s'", name, str);
 }
 
-template<> std::string BaseSetting<SandboxMode>::to_string()
+template<> std::string BaseSetting<SandboxMode>::to_string() const
 {
     if (value == smEnabled) return "true";
     else if (value == smRelaxed) return "relaxed";