This dependency version bump is in preparation for bumping the rust
toolchain to 1.94.0 (#10058). Without it, we get some compiler warnings
about conflicting `min` symbols imported from diesel:

```
warning: `min` is ambiguous
   --> nexus/db-queries/src/db/datastore/ereport.rs:22:35
    |
 22 | use diesel::dsl::{count_distinct, min};
    |                                   ^^^ ambiguous name
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #114095 <rust-lang/rust#114095>
    = note: ambiguous because of multiple glob imports of a name in the same module
note: `min` could refer to the type alias defined here
   --> /Users/dr/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/diesel-2.2.12/src/lib.rs:319:13
    |
319 |     pub use crate::helper_types::*;
    |             ^^^^^^^^^^^^^^^^^^^
    = help: consider updating this dependency to resolve this error
    = help: if updating the dependency does not resolve the problem report the problem to the author of the relevant crate
note: `min` could also refer to the type alias defined here
   --> /Users/dr/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/diesel-2.2.12/src/lib.rs:322:13
    |
322 |     pub use crate::expression::dsl::*;
    |             ^^^^^^^^^^^^^^^^^^^^^^
    = note: `#[warn(ambiguous_glob_imports)]` (part of `#[warn(future_incompatible)]`) on by default
```

Also replace deprecated `diesel::dsl::count_distinct()` with
`count().aggregate_distinct()` from `AggregateExpressionMethods`.

This mostly just picks up the Rust toolchain bump from #10058.

This mostly just picks up the Rust toolchain bump from #10058.

In #7659 I added a stupid hack to improve Helios build-and-test times in
CI: first build everything with `--exclude=omicron-nexus`, then perform
a normal build. This was to mitigate the effect of simultaneously
linking all Nexus-containing binaries (bin and test targets for both
Nexus and OMDB), which severely pushed the memory limits of Buildomat
instances at the time.

Since #10018 this job now uses an instance with 256 GB of memory. This
is enough RAM.

This change appears to shave about four minutes off of this job. This
may also have the side effect of making the build timing data useful
again. #10058 had to deal with a change to rustc's build analysis output
and given my read of the script the uploaded build timing file would
have basically only contained information about how long it took to
build Nexus. (But I could be wrong, I have not actually looked at the
file.)