Don't use read-only mode for nix build --dry-run

In dry run mode, new derivations can't be create, so running the command on anything that has not been evaluated before results in an error message of the form `don't know how to build these paths (may be caused by read-only store access)`.

For comparison, the classical `nix-build --dry-run` doesn't use read-only mode.

Closes #1795

(cherry picked from commit 54525682df707742e58311c32e9c9cb18de1e31f)

src/libcmd/installables.cc

@@ -756,7 +756,7 @@ BuiltPaths build(ref<Store> evalStore, ref<Store> store, Realise mode,
         pathsToBuild.insert(pathsToBuild.end(), b.begin(), b.end());
     }
 
-    if (mode == Realise::Nothing)
+    if (mode == Realise::Nothing || mode == Realise::Derivation)
         printMissing(store, pathsToBuild, lvlError);
     else if (mode == Realise::Outputs)
         store->buildPaths(pathsToBuild, bMode, evalStore);

src/nix/build.cc

@@ -54,7 +54,7 @@ struct CmdBuild : InstallablesCommand, MixDryRun, MixJSON, MixProfile
     {
         auto buildables = build(
             getEvalStore(), store,
-            dryRun ? Realise::Nothing : Realise::Outputs,
+            dryRun ? Realise::Derivation : Realise::Outputs,
             installables, buildMode);
 
         if (json) logger->cout("%s", derivedPathsWithHintsToJSON(buildables, store).dump());