Incorrect cached state in bst show

I encountered this error under the following conditions.

Prepare and track all elements in [buildbox-integration](https://gitlab.com/BuildGrid/buildbox/buildbox-integration)

Start building

Delete the ref for `base.bst` in the `project.refs` file

While still building in a separate terminal... type `bst show buildbox.bst`

This shows us a very suspicious state, where cache keys cannot be determined, but for some reason we are printing `cached`:

```
tristan@pony:~/work/buildbox/buildbox-integration$ bst show buildbox.bst
[--:--:--][ ][ main:core activity ] START Loading elements
[00:00:00][ ][ main:core activity ] SUCCESS Loading elements
[--:--:--][ ][ main:core activity ] START Resolving elements
[--:--:--][ ][ main:base.bst ] WARNING [unaliased-url]: tar source at base.bst [line 4 column 2]: Use of unaliased source download URL: file:///home/tristan/work/buildbox/buildbox-integration/tarball/debian.tar
[00:00:19][ ][ main:core activity ] SUCCESS Resolving elements

12 of 12 subtasks processed

[--:--:--][ ][ main:core activity ] START Initializing remote caches
[00:00:00][ ][ main:core activity ] SUCCESS Initializing remote caches
[--:--:--][ ][ main:core activity ] START Query cache
[00:00:00][ ][ main:core activity ] SUCCESS Query cache
no reference ???????????????????????????????????????????????????????????????? base.bst
cached 379e18732502e5a282fefc76b2f37250b6d2b9c8ecb3432183fe41683d6ea951 cmake.bst
cached ???????????????????????????????????????????????????????????????? glog.bst
cached ???????????????????????????????????????????????????????????????? protobuf.bst
cached ???????????????????????????????????????????????????????????????? grpc.bst
cached ???????????????????????????????????????????????????????????????? util-linux.bst
cached ???????????????????????????????????????????????????????????????? buildbox-common.bst
cached ???????????????????????????????????????????????????????????????? buildbox-casd.bst
cached ???????????????????????????????????????????????????????????????? fuse3.bst
cached ???????????????????????????????????????????????????????????????? buildbox-fuse.bst
cached ???????????????????????????????????????????????????????????????? buildbox-run-bubblewrap.bst
cached ???????????????????????????????????????????????????????????????? buildbox.bst
```

Reproduce the report with the buildbox-integration project: prepare and track all elements, remove the base.bst ref from project.refs, and run bst show buildbox.bst during the build. Compare the displayed cache states, especially the cached entries with unknown keys; done means bst show no longer reports an element as cached when its cache key cannot be determined.

Written by the indexing model from the issue text.