descriptor: don't prepend key origins twice

OriginPubkeyProvider::GetPubKey() derives its sub-provider straight into
the output provider and then prepends its origin to the entry it finds
there. The sub-providers insert with emplace(), so if the same key was
already expanded by another key expression the insert is a no-op and the
entry that gets the origin prepended is the one that already has it.

musig() is where this shows up, because it expands its participants into
the provider it is given, so two musig() expressions in the same key
expression list end up expanding a shared participant twice. A
participant origin of m/86h/1h/0h then comes out as
m/86h/1h/0h/86h/1h/0h in the input and output taproot BIP32 derivation
maps of a PSBT, so a signer that follows them derives the wrong key.
When the two expressions declare different origins for the participant,
the fingerprint of one is combined with the path of both.

Derive into a temporary provider and apply the origin there. Merge the
rest into the output, then insert the clean origin separately. This
avoids stacking paths and makes an explicit origin replace an implicit
one already present for the same key.