[remove-nixpkgs] Disable remove-nixpkgs to fix non-search fallback (#1463)

## Summary

Restores functionality implemented by
#1264 and refactored in
#1279

which was accidentally broken by
#1437

tldr: the `FillNarInfoCache` function breaks if packages are not in
search, but we support non-search packages via fallback (.e.g.
stdenv.cc.cc.lib)

I'm keeping surface area as small as possible, but as a follow up I
think we can remove the `devpkg.FillNarInfoCache` calls from `impl.Add`
if we simply ignore the`IsInBinaryCache` errors in
[ValidateExists](https://github.com/jetpack-io/devbox/blob/main/internal/devpkg/validation.go#L10-L26)
or remove `IsInBinaryCache` from that function completely (I'm not
convinced it is needed, @savil @ipince correct me if I'm wrong)

## How was it tested?
`devbox add stdenv.cc.cc.lib`

Author: mikeland73

internal/boxcli/featureflag/remove_nixpkgs.go

@@ -4,4 +4,4 @@ package featureflag
 // It leverages the search index to directly map <package>@<version> to
 // the /nix/store/<hash>-<package>-<version> that can be fetched from
 // cache.nixpkgs.org.
-var RemoveNixpkgs = enable("REMOVE_NIXPKGS")
+var RemoveNixpkgs = disable("REMOVE_NIXPKGS")

internal/devpkg/narinfo_cache.go

@@ -62,6 +62,9 @@ func (p *Package) IsInBinaryCache() (bool, error) {
 // package in the list, and caches the result.
 // Callers of IsInBinaryCache must call this function first.
 func FillNarInfoCache(ctx context.Context, packages ...*Package) error {
+	if !featureflag.RemoveNixpkgs.Enabled() {
+		return nil
+	}
 
 	// Pre-compute values read in fillNarInfoCache
 	// so they can be read from multiple go-routines without locks