riot --base option does not work with single letter URI schema

[Laura Morales]

This command

    riot --base="x:" --syntax=ttl --output=nt <( echo "<alice> <knows> <bob> ." 
)

returns this triple

    <file:///home/user/alice> <file:///home/user/knows> <file:///home/user/bob> 
.

instead of

    <x:alice> <x:knows> <x:bob> .

Riot version 4.1.0.