nix dev-shell: Make it possible to enter a profile

For example:

  $ nix dev-shell --profile /tmp/my-shell dwarffs
  (later)
  $ nix dev-shell /tmp/my-shell

1 files changed, 7 insertions, 0 deletions

diff --git a/src/nix/command.hh b/src/nix/command.hh
index d6153e42b..00c202f20 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -64,6 +64,13 @@ struct Installable
     {
         throw Error("argument '%s' cannot be evaluated", what());
     }
+
+    /* Return a value only if this installable is a store path or a
+       symlink to it. */
+    virtual std::optional<Path> getStorePath()
+    {
+        return {};
+    }
 };
 
 struct EvalCommand : virtual StoreCommand, MixEvalArgs