instances with explicit arguments in NType.agda no longer works under the master branch of Agda

[ice1000]

Building with agda 2.6.0-d6640d6:

HoTT-Agda/core/lib/NType.agda:156,7-157,47
Instance declarations with explicit arguments are never considered
by instance search, so making =-preserves-level into an instance
has no effect.

[favonia]

Thanks for the info! I will look into this when I have a little bit more time.

[rwe]

@favonia If it helps provide context, that change was introduced here: agda/agda#3419 and @jespercockx began some partial fixes here: jespercockx@a15f184

However, those fixes are incomplete. Like he commented, I'm also seeing issues in Truncation.agda and having a lot of difficulty resolving them.