fix `is_juliaup_installed` for invalid path

[t-bltg]

This PR:

• fixes https://github.com/JuliaLang/juliaup/issues/879;
• prints the Juliaup configuration file, if existing;
• ~~changes the --add_to_path description (juliainstaller --help) to match the command line argument --add-to-path (underscore -> dash)~~.

[t-bltg]

@davidanthoff, bump ?

[davidanthoff]

Hm, I'm not sure this is a good idea. If the user has a script juliaup on the PATH then things are really broken already, and I'm almost thinking we should just abort install at that point. What would be good is a better error message, for example something like "Your system has a broken juliaup command on the PATH, please first remove that" or something along those lines.

Could you move the unrelated typo fix to a different PR? Thanks!

[t-bltg]

If the user has a script juliaup on the PATH then things are really broken already, and I'm almost thinking we should just abort install at that point

Exactly. Currently, juliaup returns a false positive as exposed in https://github.com/JuliaLang/juliaup/issues/879, and is thus a bug. This PR corrects this behaviour.

[t-bltg]

Could you move the unrelated typo fix to a different PR?

Done in https://github.com/JuliaLang/juliaup/pull/916.