The treatment of the "source" parameter changed
with eb7120dc79, breaking stuff.
Before that commit, the source parameter was converted to a
string by implicit coercion, which would copy the file to the
store and yield an string containing the store path. Now, by
the virtue of escapeShellArg, toString is called explicitly on
that path, which will yield an string containing the absolute
path of the file.
This commit restores the old behavior.
9ce8df127d