fix: run gettext after dialyzer so they stop sharing _build

[jimsynz]

What

Make the gettext tool in .check.exs depend on dialyzer as well as
ex_unit, so mix gettext.extract no longer runs alongside mix dialyzer.

Why

mix gettext.extract --check-up-to-date re-expands the gettext macros by
force-recompiling the project (mix compile --force-elixir +
mix compile.elixir --force). A forced recompile empties
_build/dev/lib/bb_example_so101/ebin and .../consolidated for a second or two
before refilling them, and ex_check was running that tool in parallel with
mix dialyzer.

Elixir's build lock serialises the two compiles, but dialyzer reads beams after
it releases the lock — first when checking the PLT, then when it globs the ebin
to build its file list. If the wipe lands in either phase, dialyzer fails:

:dialyzer.run error: File not found:
  .../_build/dev/lib/bb_example_so101/consolidated/Elixir.Collectable.beam
  files: [],
:dialyzer.run error: Analysis failed with error:
No .beam files to analyze (no --src specified?)
Halting VM with exit status 1

That is what failed the bb_example_so101 job in bb's Test Subprojects
workflow on the v0.30.1 tag push (run 32619948357), and it is the same failure
tracked in beam-bots/bb#245 for bb_example_wx200. Those two are the only
packages in the ecosystem with both gettext and dialyxir, which is why only
they are affected. bb's workflow deletes _build/*/*.plt.hash before
mix check, forcing dialyxir's full PLT re-check and widening the exposure from
sub-second to roughly twenty seconds — hence the failures show up there and
essentially never here.

Verification

The mechanism was diagnosed and reproduced in bb_example_wx200
(beam-bots/bb_example_wx200#97), where mix gettext.extract overlapping
mix dialyzer reproduces the error above byte for byte, and the same one-line
change moves the forced recompile to after dialyzer finishes. This repo has an
identical tool set and .check.exs; the change here has not been run through a
local mix check of its own.

The deps: [:ex_unit, :dialyzer] restates :ex_unit because ex_check replaces
the option rather than merging it.