jigdo-file tries to parse /etc/apt/sources.list to propose a default
mirror.  Unfortunately, it trips over lines with options like the
following:

deb [signed-by=/etc/apt/trusted.gpg.d/debian-archive-buster-automatic.gpg] http://localhost:3142/debian/ unstable main non-free contrib

In particular, it proposes the mirror options as the mirror location:
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The jigdo file refers to files stored on Debian mirrors. Please
choose a Debian mirror as follows: Either enter a complete URL
pointing to a mirror (in the form
`ftp://ftp.debian.org/debian/'), or enter any regular expression
for searching through the list of mirrors: Try a two-letter
country code such as `de', or a country name like `United
States', or a server name like `sunsite'.
Debian mirror [[signed-by=/etc/apt/trusted.gpg.d/debian-archive-buster-automatic.gpg]]: