`githubRelease.unpackArchive.path` should support globbing

**Describe the problem/challenge you have**
Assets in Github releases often include the release version as part of the asset's filename, but `vendir`'s `unpackArchive` option does not currently support globbing, although the `githubRelease.assetNames` array does support globbing. Lack of globbing support means that `unpackArchive` cannot be used as intended for many Github Releases.

Currently, attempting to pass a glob results in a cryptic "Expected known archive type (zip, tgz, tar)" error message.

For example,

this works, but you have to know the filename in advance:

```yaml
- path: prometheus
githubRelease:
slug: prometheus/prometheus
latest: true
disableAutoChecksumValidation: true
assetNames: ["prometheus-*.linux-amd64.tar.gz"]
unpackArchive:
path: 'prometheus-2.25.0.linux-amd64.tar.gz'
```

this fails:

```yaml
- path: prometheus
githubRelease:
slug: prometheus/prometheus
latest: true
disableAutoChecksumValidation: true
assetNames: ["prometheus-*.linux-amd64.tar.gz"]
unpackArchive:
path: 'prometheus-*.linux-amd64.tar.gz'
```
“Error: Syncing directory ‘config/upstream’: Syncing directory ‘prometheus’ with github release contents: Expected known archive type (zip, tgz, tar)”

**Describe the solution you'd like**
Globbing should be supported. If the user-provided glob matches multiple files, an error message would be acceptable, as would be unpacking all of the specified archives.