[nexus] fix up some uses of blueprints that were using the Debug impl

[sunshowers]

Before we added the display() method, the way to get nicely formatted
blueprints was to use the Debug impl. While working on #5211, I noticed that
these callers were still using the old way to do things. Fix this up.

I tested this by making the Debug impl for Blueprint panic, then saw which
tests failed.

I also realized that we didn't have tests for omdb's blueprints. I believe at
the time it wasn't possible to write a test for this because we didn't have an
initial blueprint -- but now we do, so include a test.

(I also wanted to ensure that the actual blueprint ID was what we expected, so
I expanded the scope of redact_variable a bit. Introduce ExtraRedactions
so we can handle both fixed- and variable-length redactions, and use the same
logic for UUIDs.)

[sunshowers]

On second thought, maybe just the slice of pairs is okay. Happy to make that change if you think it makes sense.

[sunshowers]

Oh interesting... it looks like CI is failing because the length of the tmp path is different (which of course it is.) So we probably need to have two different constructors, one for fixed-length strings and one for variable-length strings. (And this justifies ExtraRedactions, a bit hard to do with just a slice of pairs!)