libcmd/installables: force re-evaluation of cached failures

I think that it's not very helpful to get "cached failures" in a wrong
`flake.nix`. This can be very confusing when debugging a Nix expression.
See for instance NixOS/nixpkgs#118115.

In fact, the eval cache allows a forced reevaluation which is used for
e.g. `nix eval`.

This change makes sure that this is the case for `nix build` as well. So
rather than

    λ ma27 [~/Projects/exp] → ../nix/outputs/out/bin/nix build -L --rebuild --experimental-features 'nix-command flakes'
    error: cached failure of attribute 'defaultPackage.x86_64-linux'

the evaluation of already-evaluated (and failed) attributes looks like
this now:

    λ ma27 [~/Projects/exp] → ../nix/outputs/out/bin/nix build -L --rebuild --experimental-features 'nix-command flakes'
    error: attribute 'hell' missing

           at /nix/store/mrnvi9ss8zn5wj6gpn4bcd68vbh42mfh-source/flake.nix:6:35:

                5|
                6|     packages.x86_64-linux.hello = nixpkgs.legacyPackages.x86_64-linux.hell;
                 |                                   ^
                7|
    (use '--show-trace' to show detailed location information)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libexpr/eval-cache.hh b/src/libexpr/eval-cache.hh
index e23e45c94..43b34ebcb 100644
--- a/src/libexpr/eval-cache.hh
+++ b/src/libexpr/eval-cache.hh
@@ -102,7 +102,7 @@ public:
 
     std::shared_ptr<AttrCursor> getAttr(std::string_view name);
 
-    std::shared_ptr<AttrCursor> findAlongAttrPath(const std::vector<Symbol> & attrPath);
+    std::shared_ptr<AttrCursor> findAlongAttrPath(const std::vector<Symbol> & attrPath, bool force = false);
 
     std::string getString();