fix: implicit string concat makes the regex bad

[edayot]

I had 50% failure on build, with a KeyError on the beet.contrig.vanilla plugin, this was due to the runtime order of the set :

{key, key.removesuffix(".*")}

Then I found this regex that used implicit string concatenation, I think I fixed it the right way.

[vberlier]

Ah, you're right this is a typo, fixed it in v0.108.2.