[remove nixpkgs] enable fallback for packages not in binary store (#1236)

## Summary

Many testscript unit-tests were failing because they use unversioned
legacy packages like `"hello"`, as opposed to `hello@latest`, or
`hello@<version>`.

These legacy packages do not use the Binary Cache Store.

I considered whether we should treat them automatically as upgraded to
`@latest`, but decided against:
1. We'll need this logic in the flake template regardless. There will be
some packages that are not in binary store.
2. This also keeps the `Package.IsInBinaryStore` easier to reason about.

We should instead encourage users to `devbox update` their packages:
which our messaging already does.

The remaining three testscript unit-tests are failing due to "unable to
find nix executable" errors. Will fix in followup.

## How was it tested?

enabled feature-flag and ran testscripts:
https://github.com/jetpack-io/devbox/actions/runs/5417704606/jobs/9848964717?pr=1236

Author: savil

internal/shellgen/tmpl/flake_remove_nixpkgs.nix.tmpl

@@ -17,6 +17,14 @@
    }:
       let
         pkgs = nixpkgs.legacyPackages.{{ .System }};
+        {{- range .FlakeInputs }}
+        {{- if .IsNixpkgs }}
+        {{.PkgImportName}} = (import {{.Name}} {
+          system = "{{ $.System }}";
+          config.allowUnfree = true;
+        });
+        {{- end }}
+        {{- end }}
       in
       {
         devShells.{{ .System }}.default = pkgs.mkShell {