There is a compromise, which will make it appear as if you are using a script.
Write a Bash function:
function miniprompt { PS1='$ ' ~/.local/bin/somescript # or just somescript if it's in $PATH }
To ensure that the function is always available, put it into ~/.bashrc
and ~/.bash_profile
(or just ~/.bashrc
if it gets loaded by ~/.bash_profile
anyway).
garyjohn made it clear that the question contains a wrong assumption that source
must be used and clarified that one could instead set PS1='$ '
directly. Thanks!