fix install support for vhs >= v0.7.2

[polds]

Charm changed how they build their artifacts to include a directory when extracted. This adds support to detect the directory and use the binary there.

Fixes #10

Manual verification:

$ asdf plugin test vhs https://github.com/polds/asdf-vhs.git "vhs --version"
Location of vhs plugin: /var/folders/l1/61srltb90tv8bsddv63lnjp40000gp/T/asdf.XXXX.hhbcB3Xfu4/plugins/vhs
Updating vhs to main
Already on 'main'
Your branch is up to date with 'origin/main'.
* Downloading vhs release 0.7.2...
vhs 0.7.2 installation was successful!
vhs version v0.7.2 (484e440)

[chessmango]

Thanks very much for the contribution! I'd been meaning to get round to this for a while <3