FOUND_CHARM is not set correctly for reactive charms

Running "charm build" for a reactive charm results in the keys and values within metadata.yaml becoming quoted.

So this:

$ grep name metadata.yaml
name: canonical-livepatch

becomes this:

$ grep name metadata.yaml
"name": "canonical-livepatch"

Which means this:

FOUND_CHARM=$(find . -maxdepth 1 -name metadata.yaml | xargs grep '^name' | awk '{print $2}')

will not find anything.

I'll put an MP together shortly, should be a trivial fix