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_wx200/ebin and .../consolidated before refilling
them. Measured locally, that window is about two seconds:

41.59 ebin=21 consolidated=0
41.70 ebin=1  consolidated=0
43.47 ebin=21 consolidated=0
43.76 ebin=21 consolidated=27

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_wx200/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

This is what has been failing the bb_example_wx200 job in bb's Test
Subprojects workflow (beam-bots/bb#245). That workflow deletes
_build/*/*.plt.hash before mix check, which forces dialyxir's full PLT
re-check — turning a sub-second exposure into a ~20 second one, so the collision
happens nearly every run there. bb_example_so101 has the same pair of tools
and fails the same way (a fix for it is on its own branch).

Verification

Locally, with BB_VERSION=local and the PLT hash deleted to match bb's CI:

• Before — mix gettext.extract overlapping mix dialyzer reproduces the
  failure above byte for byte, including the same Elixir.Collectable.beam and
  the empty files: [].

• After — mix check --no-retry puts the forced recompile in the last three
  seconds of the run, after dialyzer has finished:

  check start 32:45
  33:21 ebin=21 cons=0
  33:21 ebin=1  cons=0
  33:24 ebin=21 cons=0
  check end   33:24
  

  mix check --no-retry passes; reuse fails only on this machine, where pipx
  isn't on the asdf path.

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